Green Reading Calculator

It converts slope direction (in degrees) to radians and takes the sine of that angle to get the side-to-side component of the slope. A positive side component (angles between 0° and 180°, peaking at 90°) reads as right-to-left break, a negative one (180°-360°, peaking at 270°) reads as left-to-right, and anything within about 0.01 of zero is called straight. Because 90° and 270° are exact mirror images, getting the direction backwards flips the recommended aim point to the wrong side of the hole.

The calculator scales the raw side-slope break by a stimp factor equal to your Stimpmeter reading divided by 10 (the standard reference speed), so a 12-foot green produces 20% more predicted break than the same slope on a standard 10-foot green, and a slow 8-foot green produces 20% less. Physically this reflects that a ball rolling faster stays in motion longer over the sloped surface, giving gravity more time to pull it sideways.

Effective distance adjusts your actual putt distance for the uphill/downhill component of the slope, using a rule-of-thumb factor of roughly 10% more effective length per 1% of uphill grade, and a shorter effective distance for downhill putts. It represents how far the putt plays rather than how far it physically is, which is why an uphill 20-foot putt can show an effective distance well over 20.

The calculator starts from a baseline of 17 inches past the hole — a common guideline for how far a missed putt should roll — then adjusts it up for uphill putts by about 15% more per 1% of uphill grade (and down for downhill ones), and finally scales the result by green speed, since faster greens need less speed and therefore a softer stroke to still finish near that same window.