How Big Should Your Calorie Deficit Be? What Each Size Costs

What size deficit should you actually run?

For most people, somewhere around 10% to 20% below what they actually burn — which works out, in weight terms, at roughly 0.5% to 1% of body weight a week. The lower end of that range if you are already fairly lean, have a lot of training you care about, or have a life with dinners in it; the upper end if you are carrying more weight and the loss has a long way to run.

That is a rate, not a number for you. Nobody can hand you a personal calorie figure from a blog post, and the useful version of this question isn't "what number" anyway. It's "what does each choice cost me, and what does it actually buy in finishing date?" The honest answer is that it buys less than people expect.

The arithmetic, in plain weeks

A kilogram of body fat holds about 7,700 kcal. A kilogram is 2.2 pounds, so a pound is about 3,500 kcal. Multiply a daily deficit by seven and divide by 7,700, and you get a nominal weekly rate:

Say you want to lose 10 kg. At 500 kcal a day that is 22 weeks. At 1,000 kcal a day it is 11 weeks. On paper, the aggressive plan halves the wait, and that is why people pick it.

Why the finishing dates converge

The table assumes the deficit happens. Deficits are not run on paper, they are run across 154 individual days, and the two plans fail differently.

Work it in weekly totals instead of daily ones. Suppose the modest plan holds six days and one day lands about 500 kcal above maintenance — a long lunch, a birthday. The week nets 3,000 − 500 = 2,500 kcal, around 0.32 kg. Now the steep plan: four days at 1,000 under, three days where hunger wins and intake lands 500 over. That week nets 4,000 − 1,500 = 2,500 kcal. The same. The steeper plan bought a bigger number on the good days and paid it back on the bad ones.

I'm not going to pretend there's a published adherence percentage that makes this exact. There isn't one I can quote, and the numbers above are an illustration rather than a measurement. But the shape is familiar to anyone who has tried both: the steep plan doesn't usually end in a slightly slower average, it ends in an abandonment and a restart three weeks later. Two restarts in a twelve-week block cost more than every one of those 500-kcal lunches put together. The arithmetic of gaps is worth its own look — what a break actually costs is less than people fear and more than they'd like.

Then why not go very small?

Because a very small deficit disappears into measurement error. Logging is an estimate. A tablespoon of olive oil is about 120 kcal by USDA FoodData Central, and the difference between the tablespoon you meant and the pour you did is easily 80 kcal. Do that three times a day across oil, nut butter, cheese and the handful of cereal eaten standing up, and you have quietly mislaid 250 kcal.

Against a 1,000 kcal target, that error is a fifth. Against a 250 kcal target, it is the whole thing. A tiny deficit is not wrong, but it is unverifiable — it is smaller than the noise in your own records, and it is far smaller than the noise on the scale. Water weight moves a couple of kilograms in a day, so a rise overnight tells you almost nothing, and confirming a 0.2 kg/week trend takes many weeks of weighing before the signal separates from the wobble. Meanwhile, if your loss is the same size as your logging error, it becomes very hard to work out why nothing is happening.

So both ends fail, for opposite reasons. The middle is where the deficit is big enough to see and small enough to survive.

What a steep deficit actually costs

Four bills arrive, and none of them appears in the table.

Hunger stops being a feeling and becomes an occupation. At a modest deficit, hunger is an event before dinner. At a steep one, it is background noise all day, and it competes with everything else you were planning to think about.

Training quality goes first. Not immediately, and not dramatically — you just notice the last set is missing a couple of reps, the long run feels heavier, and you recover slower between sessions. If you are lifting to hold onto muscle while losing fat, a steep deficit is working against the thing you are lifting for.

Social eating becomes impossible rather than awkward. On a modest deficit, a restaurant meal is a large slice of the day's budget and the week absorbs it. On a steep one, a single dinner out wipes out the whole week's deficit, which means the choice is to skip the dinner or to abandon the plan. Both are bad, and repeatedly facing that choice is how deficits end. The weekly view helps here: the maths of weekends is really the maths of how much slack your chosen rate leaves.

And you compensate without deciding to. This is the one people never budget for. On a steep deficit you take the lift, you drive the short trip, you fidget less, you sit down sooner, you cancel the evening walk. None of it feels like a decision. It can quietly reclaim a meaningful slice of the deficit you were so careful to create, and the scale reads as though you have stopped losing when you have in fact stopped moving.

How to pick a rate for your own body

Use a share of your own weight, not a number you read.

Aim for roughly 0.5% to 1% of body weight per week. At 100 kg, that is 0.5–1 kg a week. At 70 kg, it is 0.35–0.7 kg. At 55 kg, it is under 0.3 kg — and anyone who tells a 55 kg person to lose a kilo a week is selling something. The percentage framing does the work automatically: the same weekly loss that is comfortable for a heavier person is a large fraction of a lighter one's total energy use.

The percentage-of-maintenance version is the same idea from the other side: take what you actually burn and come down 10–20%. That requires knowing what you actually burn, which no equation will tell you reliably — you get it from comparing a fortnight of logged intake against what the scale did. That comparison is the whole method behind finding your real maintenance, and it is what Day So Far's daily target is built from rather than from a formula about your height.

Why the right rate changes on the way down

Because both halves of the fraction move.

Maintenance falls as you get lighter — a smaller body costs less to carry and less to run. So a deficit that was 20% below maintenance at the start is 25% or more of the way through, without you changing a thing. The plan gets steeper while feeling identical, and the four bills above start arriving unannounced.

At the same time, the fat you are drawing on is less plentiful. The leaner you get, the more a large deficit asks of tissue you would rather keep. This is why the sensible path gets gentler as it goes: start nearer 1% a week if you have a lot to lose, drift toward 0.5% or below as you approach where you want to be, and recalculate maintenance every month or two rather than trusting the number you started with.

And give any change time to show itself. Weeks of flat scale readings are normal inside a working deficit, and how long a stall has to last before it means anything is longer than the fortnight most people give it. Raising the deficit in week two because the scale hasn't moved is how a 22-week plan becomes a 40-week one.

The one-line version

Pick the largest deficit you could still be running in three months' time, not the largest you can run this week. That figure is usually about half what people first choose, and it finishes at almost the same time.