A cake sinks in the middle because the centre inflated before its structure could hold, then gave way. Nine times out of ten the specific culprit is one of three things: the oven door opened before the crumb had set, there was more raising agent in the bowl than the flour could support, or the middle simply wasn’t baked through.
The dip tells you something too. A wide, shallow saucer across most of the top usually means too much lift or too much sugar. A steep-sided crater with a damp, slightly gluey band underneath means underbaked. A cake that looked perfect at the door and had collapsed by the time you turned around means it came out too early.
Here’s each cause, roughly in the order I’ve actually caused them myself.
You opened the oven door before the structure set
A sponge is a foam held up by hot air and steam until the egg proteins coagulate and the starch gelatinises. Until that happens, the whole thing is a balloon. Open the door and the temperature inside drops 15–25°C in a few seconds, the gas contracts, and the soft middle folds inward.
The structure doesn’t set until roughly the last third of the bake. So work out your bake time and don’t look before two-thirds of it has passed. In practice:
- Cupcakes and fairy cakes (18–22 min bake): nothing before 14 minutes.
- 18cm or 20cm round sandwich tins (25–30 min): nothing before 20 minutes.
- 23cm round, single deep layer (40–45 min): nothing before 30 minutes.
- 900g loaf tin (50–60 min): nothing before 40 minutes.
- Traybake in a 20x30cm tin (30–35 min): nothing before 24 minutes.
When you do open it, be quick and don’t slam. I once shut the door hard on a half-set chocolate cake and watched the middle drop about a centimetre, which I’d assumed was a myth until it happened in front of me.
There was too much raising agent in the bowl
This one surprises people, because more baking powder sounds like more rise. It is, briefly. Then the bubbles grow too large, merge, reach the surface and burst, and the cake deflates like a soufflé that’s been carried across a room.
The working standard is about 1 teaspoon of baking powder per 125g of plain flour. Self-raising flour already contains roughly that ratio, which is why adding a spoonful of baking powder “for insurance” to a self-raising recipe is one of the fastest ways to sink a cake.
An overdosed cake usually also tastes faintly soapy or metallic, and browns unevenly. If you got both the dip and the aftertaste, this is your answer.
The middle wasn’t actually baked
An underbaked centre has no set structure at all, so it sinks the moment the supporting heat is removed. The skewer test catches most of it: into the centre, out clean or with a few dry crumbs, no wet batter clinging.
But skewers lie in a chocolate cake, where a smear of melted chocolate reads identically to raw batter. So use a thermometer, which almost nobody does with cake and everybody does with meat.
- Plain butter sponges and Victoria-style cakes: 93–96°C in the centre.
- Dense, rich, high-fat cakes — chocolate mud cakes, carrot cakes, banana loaves: 88–93°C.
Below 85°C it will sink, no matter how clean the skewer looked. I started doing this after a banana loaf collapsed twice in one week; the second one read 79°C when I finally checked.
Your oven is running cooler than the dial claims
Domestic ovens are routinely 10–20°C out, and they drift as they age. Too cool and the batter spends too long in its liquid phase, rises slowly, and the middle never firms before the edges overcook and the whole thing settles.
An oven thermometer hanging from the middle shelf costs less than the ingredients for one cake. Preheat for a full 20 minutes, not the 8 minutes it takes for the light to go off, then read it.
Fan ovens are the other half of this. A fan oven runs hotter for the same dial setting, so recipes written for conventional ovens usually need dropping by 20°C — and people who get that conversion backwards end up baking at an effective 160°C when the recipe wanted 180°C. My fan oven vs conventional temperature guide has the full conversion table.
You creamed the butter and sugar for too long
Creaming forces air into the fat, and that air is your cake’s scaffolding. Overdo it and you build a scaffold heavier than the beams can carry: the batter is beautifully pale and fluffy going in, rises dramatically, and then drops.
For a standard four-egg sponge with around 225g of butter, 3 to 5 minutes on medium speed in a stand mixer is plenty. Pale, light, and holding a soft shape. Ten minutes on high is not better, it’s a different cake.
Sound counts too. Once the paddle stops making the grainy scraping noise of sugar against the bowl, you’re basically there.
There was too much sugar, or too much liquid
Sugar is a tenderiser as well as a sweetener. It grabs water, raises the temperature at which the egg proteins set, and weakens the structure. Push past roughly equal weights of sugar and flour and the crumb gets soft enough to give way under its own steam.
Liquid does the same thing by dilution. The usual causes are an extra egg because the ones in the fridge looked small, a generous glug of milk to “loosen” a batter that looked stiff, or fruit that brings its own water. Grated courgette, mashed banana and tinned pumpkin are all mostly water, and adding more than the recipe asks for is a reliable way to get a wet, sunken middle.
Weigh everything. Cup measures for flour vary by up to 20% depending on how you scoop, and that swing alone is enough to flip a good recipe.
The tin was the wrong size for the recipe
Tin size changes the depth of the batter, and depth changes everything about how heat reaches the centre.
The arithmetic is simple. Area of a round tin is π × radius².
- 18cm tin: 254 cm²
- 20cm tin: 314 cm²
- 23cm tin: 415 cm²
So a recipe written for a 20cm tin, baked in a 23cm one, spreads across 32% more area. It bakes thin, dry and pale, and often overcooks at the edges before it’s set in the middle. The same batter in an 18cm tin is 19% deeper: it climbs high, domes, the outside sets while the centre is still liquid, and then it sinks.
Rule I use: if the area differs by more than about 20%, either scale the recipe or change the time. Going bigger, drop 5–10 minutes. Going smaller, add 10–15 minutes and consider dropping the oven 10°C so the outside doesn’t run ahead. A lemon drizzle is forgiving about this. A cheesecake is not.
The baking powder was past it
Baking powder loses strength from the day the tin is opened. Six months of a slightly humid kitchen cupboard is enough to take the fizz out of it, and the expiry printed on the base assumes the tin stayed sealed.
Test it in ten seconds: 1 teaspoon into about 60ml of hot water from the kettle. It should erupt into foam immediately and enthusiastically. A lazy handful of bubbles means it’s dead — bin it.
For bicarbonate of soda, use a splash of vinegar or lemon juice instead of water; same reaction, same verdict.
A dead raising agent gives a cake that never rises much at all and settles into a dense, slightly sunken slab. This is the same failure that flattens loaves, and I’ve written about the yeast version in why your bread didn’t rise.
The batter sat on the counter before it went in
Baking powder starts reacting the moment it meets liquid. Most supermarket powders are double-acting, so a second burst comes with oven heat, but the first release is gone if the batter has been sitting.
Twenty minutes on the counter while you hunt for the tin, line it, and wait for the oven to catch up, and you’ve lost a real portion of your lift. Bicarbonate of soda is worse, because it’s single-acting: once it hits the acid in the buttermilk or yoghurt, that’s your only chance.
Preheat first, grease and line the tin first, and get the batter in within 10 minutes of the flour going in. I line my tins before I take the butter out of the fridge now, purely so there’s nothing left to do at the wrong moment.
You turned it out of the tin too soon
A cake straight from the oven is still setting. The starch continues to firm as it cools, and for the first ten minutes the crumb is fragile enough to collapse under its own weight if you tip it onto a rack.
Ten minutes in the tin on a wire rack, then turn out. Deep or heavy cakes want closer to 20. Fruit-heavy loaves and anything with a lot of syrup, longer still.
Draughts do it too. A cake cooling next to an open window, or under an extractor fan on full, cools unevenly and can dip on the cold side. Move it away from the airflow.
What to do with the sunken cake sitting on your counter
It’s not waste. A sunken cake tastes exactly the same as a flat one, and I’d argue it’s better material for three things than a perfect cake ever is.
Trifle. Cube it, soak the pieces in a couple of tablespoons of sherry, juice or coffee, and layer with custard and cream. The slightly dense band near the dip soaks up liquid better than a light sponge does.
Cake truffles. Crumble the whole cake, work in 2–3 tablespoons of cream cheese or leftover buttercream until it holds together when squeezed, roll into walnut-sized balls, chill 30 minutes, then dip in melted chocolate. This is the one I actually make. Nobody has ever asked why.
Own the crater. Fill it with whipped cream and berries, or a thick lemon curd, and it reads as a deliberate filled ring cake. Works particularly well if there’s fruit in the cake already, in which case a spoonful of the same fruit stewed down like an apple crumble filling ties it together.
And if you want to know which of the eleven causes above it was, cut a slice and look at the underside of the dip. A tight, damp, slightly translucent seam means underbaked. A dry, open, coarse crumb throughout means too much raising agent or too much air. The cake tells you, if you cut it rather than binning it.