Cake Tin Size Converter

Cake Tin Conversion Guide

Why Tin Size Matters?

The volume (and base area) of a cake tin determines how deep the batter sits and how long it takes to bake through. A recipe for a 20cm round tin used in a 23cm tin spreads thinner and bakes faster. Used in an 18cm tin, the batter sits deeper and needs longer. The base area is what determines ingredient scaling: round tin area = π × (diameter/2)². Square tin area = side × side. A 20cm round (314 cm²) vs a 23cm round (415 cm²) means you need 32% more batter (scale factor 1.32) for the same depth.

What's the difference between Round and Square Equivalences?

Converting between round and square tins: a square tin has approximately 25% more volume than a round tin of the same dimension. So a 20cm square tin ≈ 22–23cm round tin. Common equivalences: 18cm round ≈ 15cm square, 20cm round ≈ 18cm square, 23cm round ≈ 20cm square, 25cm round ≈ 23cm square. For loaf tins: a standard 2lb (900g) loaf tin (23×13×7cm) holds approximately 1.4 litres of batter — equivalent to a 20cm round tin.

What's the key thing to understand about Baking Time Adjustments?

When scaling up (larger tin): the batter is thinner, so reduce time by 10–20% and check early. When scaling down (smaller tin): the batter is deeper, so increase time by 10–25% and test with a skewer. Temperature usually stays the same. Exception: very deep cakes (Christmas cake, rich fruit cake) may need temperature reduced by 10°C to prevent burning the outside before the inside is done. Multi-layer cakes with thinner individual layers bake 20–30% faster than a single deep cake.

What's the key thing to understand about Ingredient Scaling for Baking?

All ingredients scale linearly with volume: if the scale factor is 1.3, use 1.3× every ingredient — flour, butter, sugar, eggs, baking powder, liquid. For eggs, the calculation often gives a non-whole number (1.3 eggs). Handle this by: weighing eggs (a large egg is typically 55–60g) and using the appropriate weight, or rounding to the nearest whole egg and adjusting liquid slightly, or using two medium eggs where 1.5 large eggs are needed. Leavening agents (baking powder, bicarbonate of soda) should be scaled proportionally with everything else, though very small quantities are worth rounding sensibly rather than measuring impossibly precise fractions.

What should I know about Why Baking Time Doesn't Scale Linearly with Area?

It's tempting to assume that if a new tin has, say, 20% more surface area than the original, baking time should simply reduce by a proportional amount — but heat penetration during baking depends more directly on batter depth than on surface area alone, which is why this calculator provides a specific new_time adjustment rather than a purely proportional guess based on area_new versus area_original. A wider, shallower tin holding the same total batter volume as a narrower, deeper tin will generally bake faster, because heat has a shorter distance to travel to reach the center of a shallow layer of batter compared to a deep one, even though both tins might have been calculated as holding an equivalent scaled amount of the same recipe. This is also why very large area changes (for example, converting a small round cake tin recipe to a large sheet pan) sometimes need more than a simple time scaling — the fundamentally different depth-to-area relationship can call for a genuinely different oven temperature as well as a different time, which is addressed separately in temp_note.

What should I know about Filling Depth and Oven Temperature?

A commonly used baking guideline is to fill a cake tin to roughly two-thirds full with batter, leaving enough headroom for the cake to rise without overflowing while still giving a reasonably substantial finished cake height. When converting between tin shapes and sizes, it's worth checking whether the scale_factor this calculator applies to your ingredients would still leave your new tin at roughly that same two-thirds fill level — if the new tin's shape means the scaled batter would sit notably shallower or deeper than that, the depth_note and temp_note outputs become particularly important. A noticeably deeper-than-usual fill generally bakes better with a slightly lower oven temperature and a longer time, giving the center a chance to cook through before the outside overbakes or burns; a noticeably shallower fill can usually tolerate the original or even a slightly higher temperature for a shorter time, since heat reaches the center quickly. Matching fill depth to the tin, rather than just the ingredient scale factor, is often the detail that determines whether a converted recipe bakes evenly.

What do I need to know about Converting Between Tin Shapes (Bundt, Loaf, Sheet)?

Converting a recipe between fundamentally different tin shapes — round to loaf, round to sheet, or round to bundt — needs more care than converting between two round tins of different sizes, because shape affects both how batter rises and how heat penetrates, not just surface area. The starting point is still matching volume (calculating each tin's approximate capacity in millilitres or cups, by filling with water and measuring, or using standard volume formulas for the shape), but a bundt tin's central tube means heat reaches the batter from the inside as well as outside, generally cooking faster than an equivalent-volume plain round tin, so checking for doneness earlier than a straightforward round-tin recipe would suggest is wise. A loaf tin is deep and narrow, meaning batter takes longer to bake through to the centre relative to its surface area than a wide, shallow tin of the same volume, often needing a lower oven temperature and longer time to cook through without over-browning the top and sides. A sheet pan, being wide and shallow, bakes much faster than a deep tin of equivalent total batter volume, since there's more surface area for heat to penetrate and less depth for it to travel through, typically needing a shorter baking time and sometimes a slightly lower temperature to avoid drying out such a thin layer of cake.

Tin Volume and Area at a Glance

Converting a recipe between tin shapes and sizes is really a matter of matching baking volume and surface area as closely as possible, since both affect how a batter rises and how quickly it cooks.

As a working rule, a square or rectangular tin of the same width as a round tin holds roughly 25-30% more batter, because a circle inscribed in a square only covers about 78% of the square's area — this is why swapping a round cake recipe directly into a same-width square tin without adjusting quantities usually produces a noticeably thinner cake, not an identical one.

Worked Example: 20cm Round to a 9x13in Rectangular Tin

A 20cm round tin holds approximately 314 cm² of baking area. A standard 9x13 inch (23cm x 33cm) rectangular tin holds approximately 759 cm² — roughly 2.4 times the area of the round tin. To keep the same cake thickness/depth in the larger tin, the recipe quantities (flour, sugar, eggs, butter, liquid) should be scaled up by roughly the same factor, so a recipe using 3 eggs for the 20cm round would scale to roughly 7 eggs for the 9x13 tin (759 ÷ 314 ≈ 2.4, and 3 x 2.4 ≈ 7.2, rounded to a practical whole number). Baking time typically needs reducing rather than increasing for the wider, shallower pan, because heat penetrates a thinner layer of batter faster even though the total volume is larger — a useful reminder that tin shape changes cooking time independently of the total batter volume.

Filling Depth and Oven Temperature Adjustments

When converting a recipe to a tin with a different depth-to-area ratio, adjusting oven temperature alongside baking time often gives a better result than changing time alone. A deeper, narrower tin (such as a loaf tin compared with a wide shallow sheet pan holding the same volume of batter) generally needs a slightly lower oven temperature and a longer bake, since heat has further to travel to fully cook the centre before the outside overbakes; a shallow, wide tin can usually take a slightly higher temperature for a shorter time, since the thinner layer of batter cooks through quickly. A commonly used rule of thumb is to drop the oven temperature by around 10-15°C (and expect a longer bake) whenever converting a recipe into a noticeably deeper tin than it was designed for, checking doneness with a skewer rather than relying on the original recipe's stated time.