The demand sum is straightforward: measure the area to be covered in square centimetres, multiply by the follicular-unit density you are aiming for. A hairline needs more density than a crown to look right, and the front third of a hairline needs more than the two behind it, so the multiplication is done zone by zone rather than across the whole field at once.
The supply sum is the one that binds. Take the safe donor area in cm², multiply by the safe single-pass excision density (10 to 15 follicular units per cm² at a typical baseline of 65 to 75) and check that what remains is still 40 to 50 FU/cm², the point below which the donor area starts to read as thin rather than as hair. The published example runs it end to end: 189 cm² at a baseline of 65 supports 1,890 to 2,835 grafts and leaves a residual density of 50 to 55. If your demand sum comes to 4,000 and your supply sum comes to 2,400, the answer is 2,400 and the plan changes shape. It does not become 4,000 because you would prefer it to.
One more subtraction that rarely gets mentioned at consultation: the supply figure is a lifetime figure, not a per-session one, and pattern loss is progressive. Grafts spent on a crown at 32 are not available for a hairline at 45. A plan that spends the whole donor budget in one session on the area that bothers you today is a plan that assumes your hair loss has finished. It usually has not.
