Chocolate Birthday Cake Calculator

For those times that you need chocolate cake but you can’t find a tin the right size in your cupboard.

Skip to the calculator… round cake or square/rectangular cake

For those times that you need chocolate cake but you can't find a tin the right size in your cupboard.

It seems that there’s a lot of love around for my Chocolate Birthday Cake Recipe and I’ve started to get requests for the quantities to make it in various different shapes and sizes. So far each time I’ve been asked I’ve tapped away in excel to work out what’s needed, but I thought it was about time I sorted out a calculator (just like my vanilla birthday cake calculator) so you can do it yourselves without waiting for me.

All you need to do is to enter your tin dimensions and the number of layers you’d like the cake to have into the boxes and it’ll tell you what you need. I hope you all find it useful…

Round Cake

Cake Diameter (cm)

Number of Layers

Cake Ingredients

0 g self-raising flour
0 egg(s)
0 g soft margarine or butter
0 g caster sugar
0 tsp vanilla extract
0 tsp milk
0 g cocoa powder
0 g milk chocolate

Buttercream Ingredients

0 g butter
0 g icing sugar
0 g milk chocolate
0 g cocoa powder
0 tsp vanilla extract

Square or rectangular cake

Cake Size (cm)

Number of layers

Cake Ingredients

0 g self-raising flour
0 egg(s)
0 g soft margarine or butter
0 g caster sugar
0 tsp vanilla extract
0 tsp milk
0 g cocoa powder
0 g milk chocolate

Buttercream Ingredients

0 g butter
0 g icing sugar
0 g milk chocolate
0 g cocoa powder
0 tsp vanilla extract

As with the vanilla version, I've not tested the cake in all possible sizes but as long as you don't stray too far from the original quantities it should still work well.

Chocolate Cake FAQs

What temperature should I cook the cake at?

You can cook the cake at the same temperature given in the original recipe regardless of the size of tin you're using.

How long should I bake my cake for

As long as you split the mixture into layers the cooking times will be the same as in the original recipe.

Can I cook my cake in one deep tin instead of sandwich tins?

Yes, however it will take longer to cook.

If you cook the mixture for two layers in one deep tin it will take 45-50 minutes to bake rather than the 30-35 specified in the original recipe.

I only know my tin size in inches. How can I change it to cm to use the calculator?

To convert from inches to cm, simply multiply by 2.5

Can I use this recipe to make cupcakes

Yes, I've turned this recipe into a cupcake recipe here.

This post was originally published on 17th November 2016 and updated with an improved calculator and more FAQs on 7th July 2018.


  1. Absolutely fabulous chocolate cake recipe and calculator! We made this recipe into a giant number 6 which went down very well with children and adults alike, everyone “got” that this is a truly chocolate tasing cake. Thank you

  2. Hi please could you let me know the ingredients for a 25cm cake, I can’t get the calculator to work. Chocolate cake please thank you

  3. Hi Charlotte sorry to bother you. Could you give me the correct measurements for 20cm sandwich tin, two layers using two tins.
    I have already tried your recipe and having never baked a cake before it turned out really now. I’m now wanting to a slightly smaller version of it. Thanks in advance

    • Charlotte Oates

      The original chocolate cake recipe was for a 20cm round tin so you can just follow that.

      • Lol thanks. How about 22inch? Think I got a bit mixed up 🙂

        • Charlotte Oates

          250g SR flour
          290g margarine/butter
          290g caster sugar
          5 eggs
          0.25tsp vanilla extract
          2.5tsp milk
          50g cocoa powder
          125g milk chocolate

          For the buttercream (if you’re making that too)

          290g butter
          290g icing sugar
          175g milk chocolate
          45g cocoa powder
          0.25tsp vanilla extract
          A little milk (if needed)

  4. Hi please help! I can’t get the calculator to work!
    I’ll be making a 6 inch, 2 layer choc cake
    What will I need and how long do I cook it for?

    • Charlotte Oates

      Sorry about the calculator, I’m in the process of rebuilding it to get it out of excel. In the meantime here’s the measurements you need…

      110g SR flour
      115g margarine
      115g caster sugar
      2 eggs
      1/8 tsp vanilla extract
      1 tsp milk
      20g cocoa powder
      50g milk chocolate


      165g butter
      165g icing sugar
      100g milk chocolate
      25g cocoa powder
      1/8 tsp vanilla extract
      A little milk (although you may well not need that in this warm weather).

      The cooking time and temperature will be the same as stated in the original recipe.

    • Hi Charlotte, thanks so much for your cake calculator and extremely delicious cake recipe! I made a 2 layer 24cm square cake which I baked as one cake and then cut in half and it worked perfectly – I will definitely be making this one again x

  5. Hi Charlotte,
    Thanks for the great recipes and amazing calculator! It’s a lifesaver!
    I am going to (attempt to) make a two-tier square chocolate cake (24 and 28 cm) for which I will be using your recipe.
    You mention caster sugar. I usually see just “sugar” in recipes (might be more common in Europe? not sure) so I was wondering if regular white sugar would do and whether the quantity required would be the same.
    Also, two make a two-tier cake I would assume I need 2 layers for each tier? Or is that not how it works? 🙂
    Any tips/help is much appreciated!
    Thanks again for your great advice and delicious recipes!

    • Charlotte Oates

      Hi Magda. Which country are you in? I’ll have a look at the types of sugar available there and see if I can figure out the best match for caster. Each layer is only about 3-4cm deep so you’d probably want 2 layers for each tier (or possible 3 if you want deep tiers).

      • Hi Charlotte,

        Thank you for getting back to me so quickly. I live in Germany and usually I find crystallized sugar or powdered.

        Thanks again,

        • Charlotte Oates

          I have a couple of friends who lives in Germany but are originally from England. I’ll ask them what the best equivalent is and get back to you.

  6. Hi Charlotte,
    this recipe looks amazing, I’m going to try make your cake on Saturday for my sons 3rd birthday.
    Silly question… I want to make two layers cake but I’ve only got one tin. Would it be ok to bake the first layer and then the second one? would the mixture still be ok?
    many thanks Karolina

    • You have two choices. If you have a deep tin (c. 3″ deep) then you can cook the mixture for two layers in one tin and the slice it after baking. If yuu do this it’ll take longer to cook so keep it in the over for 45-50 minutes. Alternatively you can cook each of the laters separately. If you choose to do this then you’d be best to make the mixture for each half separately just before baking otherwise you may find that the raising agents in the flour start to react with the wet ingredients while the mixture is waiting for the oven and then the cake will not rise as expected when you go to bake it.

  7. Hi Charlotte – A few weeks ago, for my Dads 60th birthday party I made your chocolate cake (Square 25X25cm) and everyone at the party agreed the cake was delicious and some even wanted more to take home with them! Thanks for a great recipe and for making me look like a pro in front of all of my family and friends!

    I noticed when looking at the calculator that the amount of ingredients for 2 layers is just twice the amount for 1, the amount of ingredients for 3 layers is just 3 times the amount for 1 etc.

    I have just looked at the calculator for a round cake this time. Firstly one with a 20cm diameter which followed the same pattern as the square cake ingredients wise – all apart from the vanilla extract which was the same amount for 1 and 2 layers and then doubles for 3 layers – is this correct or an error?

    Also I then used the calculator for a 23cm diameter round cake, I firstly looked for the ingredients for 1 layer and made a note of them, then I looked for 2 layers and the amounts weren’t the amount for 1 layer multiplied by 2 and the same with 3 layers. They all just came up with random figures. Is this actual correct or is it an error?

    Your cakes are great and I will continue to make them but I just wondered if you could clear up those couple of things because I don’t want to start weighing out wrong amounts of ingredients. Should I just take the ingredient amounts for 1 layer and double the amounts for 2 layers? Then triple them for 3 layers?

    This is a birthday cake for my friends birthday next week so if you could reply as soon as possible then that would be very very very much appreciated!! 🙂

    Thanks 🙂

    • Hi Laurel.

      Both of those things are down to rounding in the calculations to ensure that the recipe doesn’t come out with silly amounts that are difficult to measure. They sometimes don’t seem entirely logical (like the ones you’ve pointed out here) but they’re small enough that the recipe will still work well and be easier to make than having strange measurements.

      The vanilla extract is rounded to the nearest 0.25 tsp so for 2 layers it’s 0.25 (as pr the original recipe). For 1 layer it’s 0.125 which gets rounded up and for 3 layers it’s 0.375 which gets rounded up to 0.5.

      Once again for the 23cm cake the issue is down to rounding. Each time the ingredients are calculated for a different size of cake I round them to the nearest whole egg (so you don’t have a recipe calling for 3.29 eggs for example). The other ingredients are then adjusted to keep the ratios of flour, sugar, etc the same. A 23cm round cake actually needs 2.645 eggs so everything gets rounded up to allow for 3. 2 layers needs 5.29 eggs which gets rounded down to 5 eggs. 3 layers needs 7.93 which gets rounded to 8, etc. The rounding is done to ensure the recipe makes sense and all the other ingredients are adjusted to match the number of eggs which is why the ingredients for 2 layers isn’t double one layer. It works for the 20cm round cake as 2 layers uses exactly 4 eggs which is then perfectly halved to make 1 layer so no adjustments are necessary. Cakes that have been rounded up will take slightly longer to cook and down slightly less time but no a noticeable amount so checking the cake after half an hour should always be fine (as long as you’re using sandwich tins rather than making several layers in one tin which takes longer).

      I hope that all makes sense.

      • Hi I’m going to make this cake for my sons birthday it’s going to be a number cake a one and a six my tins are 40cm x 20 cm and 3 cm deep .
        I was planning on doing these times 3 cutting one in half length ways to make the number 1 and then layering the other two together and use my template to cut out the number 6 using most of the cake .
        Could you please tell me what measure of ingredients I’ll need for this please.
        He’d like a chocolate cake!

        • For the three cakes you’ll need…

          750g self-raising flour
          865g soft margarine
          865g caster sugar
          15 eggs
          1 tsp vanilla extract
          7.5 tsp milk
          150g cocoa powder
          375g milk chocolate

          You’ll probably struggle to get three cakes to cook evenly in the oven at once so you’d be best making them one at a time (due to the size, you make get way with two in at once).

  8. Hi

    I’m looking for a recipe for my sons birthday cake. The cake tin is 38x33cm. I put the measurements in the calculator (one layer) and the ingredients that come up seem a bit much… like 16 eggs? Please help! Baby’s birthday party is on Sunday!

    Thank you very much

    • Hi Riri, When I put those measurements into the calculator I’m getting 8 eggs (16 eggs with 2 layers).

      • Thank you for your reply, Charlotte. I hope you’re well.
        I’ve decided to make 2 batches of your original recipe. I’m actually going to make one chocolate cake and one vanilla cake and use both batters to make one big marble cake
        I’ll let you know how it goes!

        • I’ve not tried it as a marble cake before so I’m intrigued to know how it turns out. Let me know x

          • His Charlotte,

            So I made 2 x vanilla batter and 2 x chocolate batter to make 2 33x38cm marble layers

            I did add baking powder to the chocolate recipe.

            The cake turned out amazing, in flavour, texture and appearance!

            Thank you xo

          • That’s fantastic, I’m going to give it a try as I love marble cake 🙂

  9. Hi Charlotte,
    I’ve been searching for a chocolate cake recipe for a 10″ square tin and this looks amazing. I only have one tin and am a bit nervous about making it in one layer as a few comments suggest the cake has sunk or not risen as much as they’d hoped and I’m not too sure how long to cook it for as 45 minutes doesn’t seem long enough. Also what should I do to stop the cake burning round the edges? I was thinking of halving the ingredients and just making two cakes. Do you think this is best or should I just go for the one?

    Thank you.

    • Hi Louise,
      Personally I always bake my cake in two layers and then put them together but you should be fine to bake one deep cake. 45 minutes is probably a little low but the cake should be cooked enough to check without sinking, I’d expect it to need a few more minutes. It’s important not to overcook this cake as I find that it has less flavour if it’s been overcooked.The cake is cooked at a low temperature so you shouldn’t have any problems with it burning around the edges within that cooking time.

  10. Hi Charlotte, for the purpose of this recipe how deep would you class one layer? My cake tin in 8.5cm deep, would you say that’s 2 or even 3 layers? I’m a bit of a beginner and this page is very helpful, thank you!

  11. Hi Charlotte,

    Have you ever baked in a hemisphere tin? Im wondering about quantities for two 5 inch dome tins, to make a teddy bear’s head.
    Manu thanks

    • I’ve not tried making a hemisphere cake before so I’m not 100% confident on these measurements but I’d estimate they should be as follows for that size of tin…
      130g self-raisin flour
      25g cocoa powder
      150g caster sugar
      3 MEDIUM EGGS (this original recipe calls for large eggs but 3 large will be too much)
      65g milk chocolate
      1 1/4 tsp milk

  12. Thank you so much for the cake calculator! You really saved the day for me making my son’s 5th Birthday cake which needed to feed 20 people. I did two large rectangles as a base with a number 5 on top in black fondant which was a race track (Lightning McQueen theme). Your chocolate buttercream also worked a treat. I am so grateful for the calculator and will be recommending it to friends. Everyone loved the cake and said it was delicious!

  13. Hi Charlotte
    I’m a bit confused on the calculations for the buttercream recipe.
    In the 20cm round cake, you give equal amounts for butter and icing sugar (250g). For the square tin, you say 320g butter and 640g icing sugar. So once recipe has equal amounts of icing sugar and butter, the other recipe has double the amount of icing sugar to butter..
    Is this right?
    Thanks so much!

    • Oops, something strange happened to the icing sugar in the square calculator. It’s fixed now. It would still have worked as a recipe but you would have had a sweeter buttercream (and a bit too much of it).

      Thanks for letting me know.

  14. Hi Charlotte

    Have used this recipe for a large rectangular cake and it was a huge success. Thank you! Have you ever tried it in a Pyrex bowl to get a dome cake at all? I wanted to make a mushroom cake and for the top, I thought I might put all of the mix in one bowl rather than two tins. Do you think this would work and any idea how long it would take to bake please?

    Thanks for all your advice and ideas 🙂

    • I haven’t but I would have thought it should work, just make sure you grease it to help it come out as you can’t line the bowl as you would with a tin. The cooking time depends on the size of the bowl (the depth of the mixture). Can you tell me the diameter?

      • Sorry just seen this. I tried it and it worked beautifully. I put the whole 4 egg mixture in a large Pyrex bowl and it was in the oven at 140° for 90 mins. Used cake release and it came out easily and tasted great! Thanks again for the recipe x

  15. hi charlotte just come across your site and need to make an 8 inch (20CM) square cake in 1 layer as I only have 1 tin.
    I used your calculator for quantities which is fab.
    However I am slightly confused re cooking time. You mention timings a sper original recipe but say if baking in 2 tins in 1 bake longer. Does this mean although your calculator gives me quantities for 1 layer I should cook as if I am doing 2 in 1 i.e. for at least 45mins . Sorry I just don’t want it sinking!!!!

  16. Hi Charlotte!
    Thank you!

    • Hi Charlotte- just wanted to let you know I used a rectangular tin as you suggested and it was absolutely fine. Such a lovely cake! I’m hoping to try your vanilla one in similar sizes I also used medium eggs and it didn’t seem dry but I might try with large eggs as well- do you think it would make much difference? Thank you again for this recipe- it is really lovely!

      • Charlotte Oates

        I usually use large for this one but I have used medium if I’ve not got any large in and it’s always turned out fine.

        • Hi charlotte. I made a choclate cake 8 inch using your recipe and it went down a complete and utter delicious treat. I really wannna expand to 10 inch please could you give me the exact recipe for that x

          • If you put your cake tin dimensions into the calculator then it’ll give you the ingredients you need and then follow the usually recipe with the higher ingredients amounts.

  17. Hi Charlotte!
    I have only recently come across your website and I made your chocolate cake recipe and it was a massive success- thank you! I wanted to ask your opinion on square tints etc. I want to make an oblong cake , as close to the shape of a pint of Guinness as possible! It’s for a birthday cake! I was going to use a rectangular tin but I was conscious of the waste. Do you have any advice as to what I could use? It’s for approx twelve people. Thank you! Paula

  18. Wiil be baking a football shirt chocolate birthday cake for my grandson in a 39x26cm tin 3cm deep,possible to have mixture.

    • Hi Richard,

      I would use…
      300g flour
      345g margarine
      345g sugar
      6 eggs
      1/2 tsp vanilla extract
      3 tsp milk
      60g cocoa powder
      150g milk chocolate

  19. silly question…..but the buttercream quantities….is that enough for the middle of the cake and around?

    • It should be enough to fill the middle of the cake, cover the top and have a thin layer around the sides.

  20. Thank you so much for your chocolate cake calculator! I have the same approach to birthdays as you for my two boys. They choose the cake theme and I scour Pinterest for ideas. I need to make a Harry Potter cake next week for my 5 year old’s birthday. I have ordered a 22x32cm rectangular cake tin to make a spell book, but then just couldn’t find a choc cake recipe I trusted. Your calculator is so clever and much appreciated!! Can’t wait for the tin to arrive and have a go!

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