Making a birthday cake for 150 people is no small feat, but with the right preparation, every guest will get their piece of cake and you will keep your peace of mind. The problem of making a birthday cake to feed 150 people is really a mathematical one and it has a number of variables -- the size of the slices, the shape of the cake and the height of the cake. Once you have worked this out, it's baking time.
Decide how large you want your cake servings to be. Catering serving sizes are 5 cm (2 inches) wide, 5 cm (2 inches) deep and 10 cm (4 inches) high. To make a cake this high, you need to have two layers.
Check what size baking tin a single quantity of your recipe recommends. You can calculate the approximate quantity of cake batter you'll need and the number of servings it will make from the recommended tin size. For example, if your recipe recommends a 25 cm (10 inch) square tin, you will need to double that if you want to use a 50 cm (20 inch) tin. A square tin this size will yield roughly 100 catering-size servings once a second cake layer is added to give it a height of 10 cm (4 inches).
Choose a round or square cake. For the best visual results, a tiered cake is more appealing than a cake slab. For example, a three-tiered square cake for 150 people may have a 50 by 50 cm (20 by 20 inch) base, a 30 by 30 cm (12 by 12 inch) mid-tier and an 20 by 25 cm (8 by 10 inch) top. Don't forget that each tier is a double layer. Round cakes yield slightly less than a square tin. If making a round tier, add an extra small tier 15 cm (6 inches) in diameter to compensate for the smaller yield of servings per cake.
Bake the cakes. Set aside an entire day for making the cake. Start with the larger cakes first and mix the appropriate amount of your cake recipe for the largest tins; if your recipe calls for a 25 cm (10 inch) tin, then one 50 cm (20 inch) tin will take double the mixture. Bake according to the time stated on the recipe. Bake the subsequent tiers with the smallest baked last, mixing the cake batter as you go.
Cool the cakes. Place a base cake onto the largest cake board. Prepare your icing or filling recipe, multiplying the quantities as necessary. Place a generous amount of the filling on top of one of the base cakes, smoothing over with the icing knife. Place the second base cake carefully on top.
Place the smaller cake board or tier centred on the base cake, then position the middle cake on top, add the filling and place the second middle cake on top. Repeat with the smallest layer or tier.
Ice all the cakes and decorate as desired. When ready to serve, start from the top and work your way down.
Do a practice run with a smaller cake to test the exact height your cake recipe gives; if your cake recipe rises higher than 8.25 cm (3.5 inches), you may not need an extra layer of cake on each tier, just extra filling.