Calorie deficit: what it means and how big yours should be

What does calorie deficit mean?

Your body uses energy all day: to keep you going at rest, to digest food and to move. Food and drinks bring energy in. A calorie (strictly a kilocalorie, or kcal) is simply the unit both sides are measured in, as the NHS guide to calories explains.

When you take in less than you use, you're in a calorie deficit, and your body covers the gap from its stores. Take in more and you're in a surplus. Match the two and your weight tends to hold steady.

Two things are worth knowing from the start:

• It's an average, not a daily exam. A long Saturday dinner doesn't cancel a week. What counts is the pattern over several weeks.
• Both sides are estimates. Your energy needs come from a formula, and food values are approximations too. That's fine, as long as you treat the numbers as a starting point and let your own log refine them.

How do you calculate a calorie deficit?

Every calorie deficit calculator follows the same three steps, and Doydum shows its working in your plan's recap:

1. Resting energy. The Mifflin–St Jeor equation, published in 1990 and widely used today, estimates what you use at rest: 10 × weight in kg + 6.25 × height in cm − 5 × age, then +5 for the male formula or −161 for the female formula.
2. Daily energy. Multiply by your activity level: 1.2 (sedentary), 1.375 (lightly active), 1.55 (moderately active), 1.725 (very active) or 1.9 (extra active).
3. The deficit. Subtract the daily gap that matches your pace: weekly pace × 7,700 kcal ÷ 7.

Worked example: a 35-year-old man, 180 cm and 90 kg, lightly active, choosing 0.5 kg a week. His resting energy is 1,855 kcal. Times 1.375, that's 2,551 kcal a day. Minus 550, his target is 2,001 kcal.

Try it with your own numbers below, or start with your resting energy in the BMR calculator.

How big should a calorie deficit be?

There's no single right size, but there are limits worth respecting. Doydum offers three paces and turns each one into a daily gap with the 7,700 kcal per kg rule of thumb:

The app stops at 0.75 kg a week on purpose. A bigger gap means less food every single day, and that's where plans tend to break: hungry evenings, dinners with friends you start skipping, a plan you quietly give up on. If you're not sure, start gentle. You can always pick a quicker pace later; it's much harder to keep a plan you dread.

To see how your whole target compares with your needs, our guide on how many calories to eat a day walks through the full picture.

Is 7,700 calories really 1 kg?

The 7,700 kcal figure is a handy average for the energy stored in a kilogram of body fat, and it's how Doydum turns a weekly pace into a daily number. But it's a simplification, and a well-known one.

Work by Hall and colleagues (The Lancet, 2011) showed why the simple sum overpromises: as your weight changes, your energy needs change too, so real change slows over time. A gap that started at 550 kcal a day quietly gets smaller.

What this means in practice:

• Treat the weekly pace as a direction, not a schedule.
• Expect the scale to wobble. Day-to-day swings often come from water and the food still in your system, so look at the trend over several weeks.
• If progress stalls for weeks, check your log and your numbers again rather than cutting harder.

What is the lowest calorie target Doydum allows?

However the maths turns out, Doydum never sets your target below 1,200 kcal a day for the female formula or 1,500 kcal for the male formula. If your numbers would land lower, the target holds at the minimum and your pace on paper simply becomes slower.

These are estimates from a formula, not a prescription, and needs differ. Doydum is made for adults. If you're under 18, pregnant or breastfeeding, have a medical condition or a history of eating disorders, talk to a doctor or registered dietitian before aiming for any deficit.

How to create a calorie deficit without overhauling your life

You don't need special products or a list of banned foods. A deficit comes from small, repeatable changes to what you already eat:

• Look at portions first. The same meal on a slightly smaller plate is often the easiest gap you'll ever make.
• Notice your drinks. Sweet drinks and milky coffees add up quietly over a day.
• Lean on filling, lighter dishes. Red lentil soup is about 55 kcal per 100 g, and soups, vegetables and legumes fill a plate generously.
• Keep the foods you love. A piece of baklava at a family dinner fits into a week. Logging it honestly is what keeps your numbers useful.
• Keep it simple when you eat out. Sharing a side or asking for a half portion changes the numbers, not the pleasure of the meal. More ideas in counting calories when eating out.
• Move a little more if you like. Walking adds to the energy you use, and in Doydum your steps have their own goal on Doy's farm.

How do you know you’re really in a deficit?

A formula can only guess; your log shows what actually happens. After two or three weeks of logging most days, your calorie chart shows your real average intake and your weigh-ins show the trend. Put the two side by side and you'll know more than any calculator can tell you.

• Log most days, not perfect days. A rough entry beats a blank one. In Doydum, days you didn't log show as unlogged, never as zero, so a forgotten day won't fake a huge deficit.
• Read 30 or 90 days, not one. Weekly ups and downs even out over a month.
• Ask for a second pair of eyes. Tap Ask Doy on a chart and Doy reads it the way an experienced registered dietitian reads a client's log, with dates and a few small next steps. It's AI-written and not medical advice.

New to logging? Start with how to count calories, or keep a simple food diary first.

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