The order these pages go in
This cluster is a sequence, not a menu, and using it out of order is the most common way to get a wrong answer from a correct calculator.
Maintenance first. Every other number here is maintenance plus or minus something, so an error in it propagates into all of them at full size. Then the direction: a deficit if you are cutting, a surplus if you are gaining, or maintenance itself if you are recomposing. Then protein, which is set from bodyweight and does not change when the calorie total does. Then the split, which distributes whatever calories are left between fat and carbohydrate. And when the cut is over, the climb back, which is the phase almost nobody plans and the one where a hard-won deficit is most often given back.
The arithmetic, in full
Maintenance = BMR × activity factor. Cutting calories = maintenance − deficit. Bulking calories = maintenance + surplus. Weekly rate = daily change × 7 ÷ 7,700 kcal per kilogram of fat, which is the familiar 3,500 kcal per pound. Protein = basis weight × a coefficient set by your goal. Fat = calories × a share, with a floor of 0.5 g per kilogram. Carbohydrate = whatever is left. There is nothing else in here, and any site presenting this as proprietary is selling you arithmetic.
The deficit and the surplus are not symmetrical
This is the single most useful thing to know before using any of these pages. A 500 kcal deficit is an ordinary, defensible cut. A 500 kcal surplus is the aggressive option on the bulking page and for most trained lifters it is mostly fat.
The reason is that the two processes have different limits. Fat loss is limited by how much stored energy the body will release per day, which for most people is a great deal. Muscle gain is limited by how quickly tissue can actually be built, which is very slow — on the order of a quarter to half a kilogram a month for a trained lifter, and less as training age rises. Calories above that ceiling do not become extra muscle.
Three things that reliably go wrong
The activity multiplier. Most people pick one level too high, and one step on that scale is worth around 300 kcal — nearly three times the disagreement between the BMR equations everyone argues about. If you have a desk job and train four times a week, you are ‘moderate’, not ‘very active’.
The horizon. A cut planned entirely on week-one arithmetic runs behind schedule, because maintenance falls as you get lighter and a fixed calorie target becomes a smaller deficit every week. Recalculate every 4 to 6 kg.
Reading the scale weekly. Water shifts from carbohydrate intake, sodium and hormonal cycles routinely mask a fortnight of genuine fat loss. Weigh daily, average over seven days, and compare averages. A single morning reading is noise with a decimal point.
How accurate is any of this
Maintenance is uncertain by roughly ±10% before you have gathered any of your own data — that is around 275 kcal on a 2,750 kcal estimate, which is larger than most of the decisions you will make on top of it. The honest way to use these pages is as a starting hypothesis: take the number, hold it for two weeks, weigh daily, and let your own trend correct the estimate. After that you are working from a measurement of you rather than a regression fitted on somebody else.
Nothing in this cluster is a diet plan, a medical assessment, or a judgement about what you should weigh. Every page states its formula and its limitations, and every calorie output on this site carries a floor and a referral, because a number that tells somebody to eat very little is the one place a calculator can actually do harm.