Bulking and cutting: how big a surplus, deficit and weekly rate

What bulking and cutting are

They are the two classic phases of body-composition-oriented training. In the bulking phase you eat above your maintenance to give your body spare energy and material to build muscle with; you accept you will gain some fat along the way. In the cutting phase you eat below your maintenance to lose that fat and reveal the muscle you have built, protecting it with high protein and strength training. The idea is simple: first you build, then you polish.

Bulking and cutting numbers at a glance

These are the numbers proposed by the two reference reviews for natural bodybuilders — Iraki et al. (2019) for the bulk and Helms et al. (2014) for the cut — and by the trials behind them. Percentages are calculated on your maintenance, which you can estimate with the calorie calculator:

Worked example: 75 kg and 2,600 kcal maintenance

To see how those percentages translate, take a 75 kg person at 18% body fat (13.5 kg of fat and 61.5 kg of lean mass) whose estimated maintenance is 2,600 kcal per day:

• Bulk: 2,600 + 10% = 2,860 kcal/day. Target pace: 0.25-0.5% of 75 kg = 0.19-0.38 kg per week (about 0.8-1.6 kg per month). Protein: 1.6-2.2 g/kg = 120-165 g/day.
• Cut: 2,600 − 20% = 2,080 kcal/day. Target pace: 0.5-1% of 75 kg = 0.375-0.75 kg per week. Protein: 2-2.4 g/kg = 150-180 g/day.
• How long the cut lasts: going from 18% to 12% body fat while keeping the 61.5 kg of lean mass means reaching about 69.9 kg (61.5 ÷ 0.88), i.e. losing ~5.1 kg. At 1% per week that is about 7 weeks; at 0.7%, about 10; at 0.5%, about 14.
• How much a bulk adds: 16 weeks at 0.19-0.38 kg per week is 3-6 kg on the scale, and not all of it will be muscle.

Why they are separated into phases

Because building muscle and losing fat call for opposite energy signals. An energy surplus has an anabolic effect of its own, although nobody has yet pinned down the ideal size of that surplus (Slater et al., Frontiers in Nutrition, 2019); fat, on the other hand, is only lost in a deficit. In beginners and people with fat to spare the body can do both at once — that is body recomposition — but as you gain experience and get leaner, that margin narrows. That is why people who have been training for years usually alternate: a few months building, a few months cutting.

How to know which one you need

The practical rule is to look at your starting body fat, not your mood. These thresholds are a guide, not a physiological limit:

• Men below 15% / women below 25%: you have room for a bulk.
• Men above 20% / women above 30%: cut first.
• In the middle zone: either works; choose by your nearest goal.

The bulking phase: how to do it right

A good bulk is boring and controlled, not a feast. Add about 10% to your maintenance — roughly 200-300 kcal if you maintain on 2,000-3,000 kcal; the muscle-gain calorie calculator gives you the number. Iraki et al. (2019) propose 10-20% for novices and intermediates and somewhat less for advanced lifters, with a pace of 0.25-0.5% of your bodyweight per week. Going faster does not pay off: in the trial by Helms et al. (2023), with trained lifters over 8 weeks, moving from a 5% to a 15% surplus mainly added fat, with no clear evidence of more muscle (only the bench press improved a little more). Keep protein at 1.6-2.2 g/kg: the meta-analysis by Morton et al. (2018) found no further lean-mass gain, on average, above ~1.6 g/kg. And strength train with progressive overload: without that stimulus the surplus goes to fat. Adjust the split with the guide on how to calculate your macros and review what to eat to gain muscle.

The cutting phase: how to do it right

The goal is to lose fat while keeping muscle, and the number that rules is the pace: 0.5-1% of your bodyweight per week (Helms et al., 2014). To get there, start with a deficit of about 20% — roughly 400-600 kcal if you maintain on 2,000-3,000 — and adjust according to the scale; the weight loss calculator gives you the starting point. In the trial by Garthe et al. (2011), with 24 elite athletes doing 4 strength sessions per week, the group that cut energy intake by 19% lost 0.7% per week and gained 2.1% lean mass; the group that cut 30% lost 1% per week and gained nothing (−0.2%), with the same total weight loss (~5.5%). Raise your protein: Helms et al. (2014) recommend 2.3-3.1 g/kg of lean mass, and Longland et al. (2016) found that during a 40% deficit for 4 weeks, 2.4 g/kg gained 1.2 kg of lean mass versus 0.1 kg with 1.2 g/kg. Maintain (do not increase) your strength training: it is the signal that tells your body to keep the muscle. Properly understand the calorie deficit before you start.

How long each phase lasts and how to switch between them

No study sets the length of a bulk: muscle is built slowly, so it usually lasts months, and the sensible approach is to end it when your waist or your estimated body fat approaches the threshold at which this guide would tell you to cut. A cut lasts as long as it takes to reach your target body fat at 0.5-1% per week: in the 75 kg example, losing ~5 kg takes about 7-14 weeks, and in Garthe et al. (2011) the slow group needed 8.5 weeks on average to lose 5.6% of their bodyweight. The transition matters as much as the phases: when a cut ends, your expenditure is already lower than at the start — you weigh less and your body adapts downward in a deficit (Trexler et al., 2014) — so first raise calories to your new maintenance and do not jump straight into a surplus. Learn to avoid that spike in how to avoid the rebound effect.

Clean bulk vs dirty bulk

The "dirty bulk" — eating with no control assuming everything turns into muscle — is one of the most common mistakes. Garthe et al. (2013) measured it in elite athletes over 8-12 weeks: the group on a surplus meal plan ate about 620 kcal more per day than the group eating freely (3,585 versus 2,964 kcal) and gained more weight (3.9% versus 1.5%) and much more fat (+15% versus +3%), but no more lean mass. Helms et al. (2023) point the same way. A bigger surplus is not more muscle: it is more fat that you will later have to remove in a longer cut. A clean bulk lets you cut for less time and look good for more months of the year.

How to measure each phase

Each phase is measured differently. In a bulk you want the scale to climb slowly and your gym lifts to climb too; if your waist balloons, you are gaining too much fat and it is time to trim the surplus. In a cut you want your weight to drop slowly and your lifts to hold; if your strength collapses, the deficit is too aggressive. In both, trust your weekly average weight, your waist, your lifts and photos every 4 weeks, not the daily number. Read how to measure your progress beyond the scale.

In both a bulk and a cut, success depends on getting your calories and protein right day after day for months — and that is where almost everyone gets lost. With Renzy you photograph your food and see in about 20 seconds whether you are above or below your target for the day and whether you hit your protein, without weighing anything or writing anything down. That daily reference is what keeps you on course long enough for the phase — whichever one it is — to work.