Grade Adjusted Pace
Calculator

It converts your actual pace on hilly terrain into the equivalent effort pace on flat ground, accounting for the extra energy cost of climbing and the reduced (but not free) energy savings of descending, so you can compare hilly and flat efforts on a fair, effort-based basis.

While downhill running is generally faster and less metabolically costly than flat running, it's not proportionally as beneficial as uphill running is costly, particularly at steeper grades where eccentric muscle braking increases effort again, so a hilly course with equal up and down is still typically harder than a flat one of the same distance.

Most grade-adjusted pace formulas apply a nonlinear cost curve based on percent grade, since the energy cost of a given incline increases faster than linearly as grade steepens, and some models also account for the diminishing (and eventually reversing) benefit of very steep descents.

It lets you evaluate whether a hilly run was actually a quality effort even if your raw pace looks slower than a flat-course run, and it allows fairer comparison of training consistency across routes with different elevation profiles rather than judging purely by raw pace.

It gives a reasonable estimate of equivalent flat effort, but it doesn't fully capture race-specific factors like pacing strategy, taper, and race-day conditions, so it's best used as a training tool for comparing effort across runs rather than a precise flat-race time predictor.