Kitchen math

Halving a recipe with 3 eggs, and other kitchen math nobody taught you

Half of three eggs is one and a half, and you can measure it exactly. Here's the kitchen math for halving anything: odd fractions, pan sizes, timing and the method.

By Rob Bush · Updated 4 Oct 2026 · 7 min read

It's a Tuesday night and the brownie recipe makes sixteen. There are two of you in the house, and neither of you has ever stopped at one brownie, so you decide to halve it. Everything goes beautifully right up until the eggs. The recipe wants three. Half of three is one and a half, and nobody has ever sold half an egg.

This is the moment most people either throw in two eggs and hope, or make the full batch and promise to freeze half. (Nobody freezes half.) There's a better way, and it takes about a minute. Here's the kitchen math that recipes assume you already know.

What's half of three eggs?

One and a half eggs, and you can measure it exactly.

Crack an egg into a small bowl and beat it until the yolk and white are one even yellow, with no slippery streaks of white. Now it's a liquid, and liquids can be measured. According to the USDA's FoodData Central, a large egg out of its shell weighs about 50 grams, and a cup of beaten egg is about 4.86 large eggs. Do the division and one large egg comes to roughly 3¼ tablespoons.

So, for half of three eggs:

1. Add one whole egg to the bowl.
2. Beat a second egg in a separate cup.
3. Add half of it: 25 grams on a scale, or about 1⅔ tablespoons (1½ tablespoons plus ½ teaspoon, if you like precision).

The other half goes into tomorrow's scrambled eggs or gets brushed over a pie crust. Halving a recipe with one egg works the same way: beat it, use half, keep the rest covered in the fridge.

Why does "large" matter? The American Egg Board calls large the standard size for recipes and cooking. If a recipe just says "eggs," it means large. If your carton says jumbo, a scale is your friend: USDA data puts a jumbo egg at about 63 grams, so three jumbo eggs are nearly four large ones.

The halving cheat sheet

The rest of halving is fractions, and fractions are where things go sideways at the counter, usually with a measuring cup in one hand and a phone in the other. Two facts sit under everything: a cup is 16 tablespoons, and a tablespoon is 3 teaspoons. Every line below comes from those two.

The two worth a sticky note inside a cabinet door are ⅓ cup and ¾ cup, because no measuring cup in the drawer is exactly half of either.

And if a recipe gives weights, halving is just dividing by two. No spoons, no fractions, no squinting at the side of a measuring jug. That's the whole case for a kitchen scale, in one sentence.

Pan math: why 8×8 is "half" of 9×13

A 9×13 pan has 117 square inches of floor. An 8×8 has 64. That's 55 percent, close enough to half that the 8×8 is the standard swap, and it's why so many halved brownie and casserole recipes end up there.

It isn't exactly half, though. Half the batter spread over 64 square inches, instead of the 58½ it would get at the original thickness, sits about nine-tenths as deep. Thinner bakes faster. Remember that for the next section.

A few more floors worth knowing:

• 9-inch round: about 64 square inches, the same as an 8×8
• 8-inch round: about 50 square inches
• 9×9 square: 81 square inches

So half a 9×13 cake can also go into a 9-inch round. What you want to avoid is a pan that's badly off in either direction. Too big, and the batter spreads into a thin, dry sheet. Too small, and you get a deep middle that's still raw while the edges turn to roof tiles.

Cooking time doesn't halve

This one catches nearly everybody. Halving the ingredients doesn't halve the time, because heat doesn't care how much food there is. It cares how far it has to travel to reach the middle.

• Cookies, muffins, biscuits: same time. Each one is its own little unit. America's Test Kitchen makes this point about doubling, and it holds in reverse.
• Brownies or a casserole in the 8×8: a bit faster, because the layer is slightly thinner. Start checking at about two-thirds of the original time.
• A cake in a smaller pan at the same depth: close to the original time, maybe a few minutes less.
• Soup, stew, chili: almost no change. Half a pot comes to a simmer sooner, but tough meat and dried beans soften on their own schedule.
• A roast: a smaller piece cooks faster, but by how much depends on its thickness, not its weight. Go by a thermometer, not the clock.

The oven temperature never changes. 350°F (177°C) stays 350°F. Nobody would actually halve it, but it's worth saying, because it's exactly the kind of number that gets "helpfully" halved when someone runs a whole recipe through a calculator.

The working rule: check early, and trust the doneness cues the recipe gives you over the timer. A toothpick with a few moist crumbs, a center that barely jiggles, edges that bubble. Those were written for exactly this situation.

The instructions are part of the recipe

Here's the trap nobody warns you about. You halve the ingredient list carefully, jot it on the back of an envelope, start cooking, and then step four says:

Add both eggs and beat until pale.

Both? You have one and a half. Your brain, already busy with a hot pan and a timer, briefly forgets you halved anything at all. Most of us have added the wrong amount at exactly this point.

Amounts hide in the method all the time:

• "Add both eggs"
• "Pour in half the stock" (half of the recipe's stock, or half of yours?)
• "Reserve 1 cup of the pasta water"
• "Spread into a 9×13 pan"
• "Divide into 12 balls"
• "Bake for 40 minutes"

Before you start, read the method with a pen. Every time it names an amount, a count, a pan or a time, write the halved version above it. It takes two minutes, and it rescues the one moment you'd otherwise be guessing with wet hands.

Old recipes are the worst offenders, because they lean on packaging. "One box of cake mix." "A can of evaporated milk." Those packages haven't always stayed the same size (that's the shrinkflation problem), so work out what "one can" meant when the card was written before you halve it.

Seasoning and leavening in small batches

Baking is chemistry, so baking powder, baking soda and salt get halved exactly, just like the flour. The trouble is that tiny amounts are easy to get wrong. A heaped quarter teaspoon can hold far more than a level one, and in a half batch that error is a bigger share of the whole. Level every spoon with the back of a knife. For ⅛ teaspoon without the right spoon, fill a ¼ teaspoon level and push half of it off.

Savory cooking is more forgiving. Halve the salt and spices, taste near the end, and adjust. With garlic, chili and anything fierce, start a little under half. You can always add more. You can never take it out.

A few things don't halve at all:

• The film of oil that coats the pan. A skillet needs it whether you're cooking two chicken thighs or eight.
• Water for boiling pasta or blanching vegetables. You need enough to keep a rolling boil when the food goes in.
• "Season to taste," "a pinch," "a splash." They were never measured to begin with.

Going the other direction has its own surprises, by the way. Here's what changes when you double a recipe.

Try it tonight

Pick one recipe you always make too much of, the lasagna that feeds eight when there are three of you, or the cookies that vanish too fast. Halve it on paper before you cook. Do the ingredient list with the cheat sheet above. Then go through the method with a pen, line by line, and halve every amount, count and pan it mentions. If there's an odd number of eggs, beat one and measure.

Then write the halved version right on the card or the printout, so next time the math is already done.

Rob Bush is the founder of A La Carte.