The short answer
A score differential is the per-round number the World Handicap System uses to track how you played, relative to the difficulty of the course and tees. Your Handicap Index is simply the average of the best 8 of your last 20 differentials.
The formula, explained
Every differential comes from the same equation:
(113 / Slope Rating) × (Adjusted Gross Score − Course Rating − PCC)
Reading it left to right: Adjusted Gross Score is your score with any blow-up holes capped at a Net Double Bogey. Course Rating is what a scratch golfer is expected to shoot, so subtracting it tells you how far above or below that standard you played. Slope Rating over 113 scales that gap to a standard difficulty, so hard courses count for more. PCC is a small daily conditions adjustment, usually zero. The result, rounded to one decimal, is your differential for that round.
Two rounds, same score, different differentials
This is the idea that makes differentials click. Imagine you shoot a gross 88 twice, at two very different courses.
At Oakland Park off the white tees (Course Rating 66.3, Slope 112), an 88 works out as (113 / 112) × (88 − 66.3) = 21.9. At The Heron Country Club off the white tees (Course Rating 73.4, Slope 131), the very same 88 is (113 / 131) × (88 − 73.4) = 12.6. Identical scorecard total, but one was a far better golfing performance, because the course was much harder. The differential is what captures that difference; your gross score alone never could.
What is a “good” differential?
A rough feel: a scratch golfer posts differentials hovering around 0, a 10 handicap produces differentials around 10, and a 20 handicap around 20. Your differentials will scatter, some good days, some poor, which is exactly why the system only keeps your better ones. A single low differential does not drop your handicap on its own; it has to earn its place among your best 8.
See your differential for any course, instantly
Real course ratings, no spreadsheet.
How differentials feed into your handicap index
Your Handicap Index is the average of the lowest 8 differentials in your most recent 20 rounds. As each new round comes in, the oldest drops off, so the index is always a rolling picture of your recent better golf. Two extra guardrails keep it sensible: a soft cap slows sharp rises, and a hard cap limits how far it can climb in a year. For the full mechanism, see how the WHS handicap is calculated.
Common confusions
Differential vs Handicap Index. A differential is one round; your index is the average of your best 8 differentials. Differential vs Playing Handicap. Your Playing Handicap is the number of strokes you actually receive in a given competition, worked out from your index for the specific tees and format. The differential is upstream of all of that, it is purely a measure of one round.