How big should a bulking surplus be?

About 200 to 250 kcal a day for a trained lifter. A pound of lean tissue costs roughly 2,500 kcal — far less than a pound of fat at 3,500, because lean tissue is mostly water — so a 0.5 lb per week gain needs only 179 kcal a day of usable surplus.

Why am I not gaining muscle even though I eat a lot?

For a hybrid athlete the usual answer is that the surplus is not real. If maintenance was estimated without your endurance work in it, three runs a week adds 300 to 500 kcal a day of expenditure and nothing was added on the intake side. You are at maintenance, or below.

How many calories to build a pound of muscle?

Roughly 2,500, against about 3,500 for a pound of fat. Adipose tissue is around 87 percent lipid at 9 kcal per gram; lean tissue is mostly water, and the dry protein in it carries 4 kcal per gram.

Is a 500 calorie surplus too much?

For a trained lifter, yes. At an intermediate lean-gain ceiling of 0.5 lb per week the usable surplus is 179 kcal a day, so 321 a day has nowhere to go but fat — about 0.64 lb of fat a week, or ten pounds across a sixteen-week block.

How do I know if my surplus is real?

Measure your maintenance from your own trend weight against your own logged intake over fourteen days, then compare what you are eating to it. That figure already contains every calorie your training cost, whether or not you logged the sessions.

The 179-Calorie Problem

Where 179 comes from

0.5 lb/week × 2500 kcal/lb ÷ 7 days = 179 kcal/day

The 2,500 is the part people find surprising. Adipose tissue is roughly 87% lipid, and lipid carries about 9 kcal per gram — which is where the familiar 3,500 kcal per pound of fat comes from. Lean tissue is mostly water. The dry protein in it is a fraction of the mass and protein carries 4 kcal per gram.

Muscle is cheap. It is just slow.

The ceiling falls fast

Training age	Realistic lean gain	Daily energy needed
Untrained, first year	~1.0 lb/week	~357 kcal
Intermediate, 2–4 years	~0.5 lb/week	~179 kcal
Advanced, 5+ years	~0.25 lb/week	~89 kcal
Near genetic potential	~0.125 lb/week	~45 kcal

The rate is capped by the training stimulus, satellite cell activity and protein synthesis — not by available energy. Eating more does not make it faster. It makes the excess go somewhere else.

What a +500 surplus is actually buying

If your ceiling is 179 and you eat 500 over maintenance, 321 kcal a day is surplus to requirements in the most literal sense.

321 × 7 ÷ 3500 = 0.64 lb of fat per week

Over a sixteen-week block that is ten pounds of fat you did not need, bought at the price of a cut afterwards that will cost you some of the muscle you just built.

The calculator does this for your own numbers.

Why this is sharper for hybrid athletes

Every figure above assumes the surplus is real. For a hybrid athlete it frequently is not, and the arithmetic of why is brutal.

Session	Cost for a 180 lb athlete
Easy 40-minute run	~625 kcal
Tempo 45 minutes	~750 kcal
Long run, 12 miles	~1,500 kcal
Three easy runs a week	~270 kcal/day

An advanced lifter's entire surplus is erased by less than one mile of running. An intermediate's is erased by three easy runs a week.

If your maintenance figure was estimated in a week when you were not running — or came from a calculator with an activity dropdown, which is worse — then adding endurance training silently moved your expenditure by several hundred calories a day and moved your intake by nothing.

You are not in a surplus. You are at maintenance, or in a deficit, wondering why the squat has stalled.

The diagnosis is always wrong from the inside

Three conclusions people reach, in order of frequency:

"My metabolism is broken." It is not. It is higher than the number you were eating against.

"Interference is real after all." Possibly at the margin. But interference does not stop the scale moving. Energy balance does.

"I need to eat more protein." Usually you already do. Protein is not the binding constraint when total energy is.

The actual answer is that a number produced without your training in it was used to plan a life that had your training in it.

Why arithmetic cannot fix this

Here is the part that matters, and it generalises past this specific problem.

You cannot steer at 89 calories using a method whose error bar is 400 calories.

Every prediction of your expenditure inherits compounding error: a prediction equation fitted on general populations, an activity multiplier that flattens your training week, a wrist device estimating your running economy without knowing it, and non-exercise movement that is the largest term and completely unmeasurable from a form.

Stack four estimates and the compound error is easily several hundred calories. The signal you are trying to detect is 89. That is not an imprecise measurement — it is a measurement that cannot resolve the quantity at all.

What can resolve it

Not adding up your expenditure. Observing the outcome.

weightChangeLb = trendWeight(today) - trendWeight(14 days ago)
energyBalance  = weightChangeLb * 3500 / 14
maintenance    = avgDailyIntake - energyBalance

Your bodyweight is an integrator. It sums every calorie in and every calorie out, including the ones nobody measured — the run your watch under-estimated, the fidgeting you did not notice, your particular running economy, your particular metabolism.

Fourteen days of honest logging and eight weigh-ins, and you have a maintenance figure that needs no assumption about any of it. The full method and its failure cases.

The trend on the scale can resolve 89 calories a day over a fortnight. No arithmetic you could do can.

The general lesson

This is the reason so much of the app is built on measured rather than predicted quantities, and it is not an ideological preference.

When the signal you care about is smaller than the error of the method you are using to find it, you need a different method — not a more careful application of the same one.

Maintenance calories are the clearest example. Threshold pace is another: estimated from your own efforts rather than from a table. Recovery is a third: scored against your own thirty days rather than against a population.

In each case the alternative was available, conventional, and unable to resolve the thing that mattered.

What to do next

Work out your own ceiling from the table above, then compare it against your weekly endurance volume converted to a daily figure. For most hybrid athletes the second number is larger than the first, and that single comparison explains a year of confusion.