Slope-Adjusted Playing Distance
A laser reads the straight line to the flag. Uphill, that line is shorter than the ground you have to cover and the ball has to climb as well; downhill, the opposite. This works the geometry — true horizontal distance, elevation change and the plays-like number — and states plainly when Rule 4.3a lets you use it.
The raw number a laser gives you with slope switched off.
Uphill positive, downhill negative. This is the inclinometer reading a slope-capable device takes.
Rule 4.3a(1) — read this before a counting round
A distance-measuring device may be used to measure distance only. Using it to gauge elevation change — the slope function — breaches Rule 4.3: the general penalty for a first breach (two strokes in stroke play, loss of hole in match play), disqualification for a second. A device that has a slope function is still allowed provided the function is switched off, and committees routinely expect the disabled state to be visible on the device. A committee can also ban devices outright under Model Local Rule G-5, in which case none of the above helps you. Check the notice board, not the box the device came in.
Practice rounds, casual golf and yardage-book work are exactly what this calculator is for. How we grade a device on its lock-out mechanism and its acquisition behaviour is set out in our methodology.