Three weeks is the most common length in marathon plans, with two weeks used by runners on lower volume and four by those on very high mileage. This calculator supports all three. The point of the taper is to arrive rested without losing fitness, and the evidence-backed principle underneath every version is the same: cut volume, keep some intensity.
The convention this calculator uses is roughly 80% of peak in the first taper week, 60% in the second and 40% in race week, excluding the marathon itself. Those are round numbers from common practice rather than a measured law, so treat them as a starting shape rather than a prescription. What matters more than the exact percentage is that the reduction is real and that it starts from your genuine peak week.
No. Cutting volume while keeping some race-pace or threshold work is the standard approach, because dropping intensity entirely is what makes runners feel flat on race day. Sessions get shorter, not softer. A common race week has one short session with a handful of race-pace kilometres, several days out from the marathon.
This calculator scales the long run down with weekly volume, which typically puts the final genuinely long run two to three weeks out from race day. The runs after it are shorter and exist to keep the legs turning over rather than to add fitness. Nothing you run in the last fortnight makes you fitter for the marathon, but plenty of it can make you tired.
Taper flu, the heavy-legged and slightly anxious feeling that arrives when training volume drops, is common and usually passes in the last few days. It is not evidence that the taper is wrong. If you want to check the other half of the plan, the mileage ramp calculator builds the weeks that lead up to your peak.