Type in the readings for each cylinder. The calculator gives the spread as a share of the highest reading and marks every cylinder more than 10% below it. Two low cylinders next to each other in the numbering get a flag of their own.
Reading the results
Yachting magazine makes the point that there's no single right figure: different sizes, makes and models produce different pressures. What matters is the variation, and all cylinders should read within about 10% of each other. The calculator measures each cylinder against the highest.
If the maker publishes a minimum, enter it too. The service manual or a dealer will have it for a given engine.
What low readings mean
Yachting magazine's list: a scored cylinder, worn rings, a broken ring (likely with a scored cylinder too), or carbon making the rings stick. Two low cylinders next to each other point strongly to a failed head gasket between them. Check the engine's cylinder numbering before you call two cylinders adjacent, since numbering differs between makers.
Taking the test
Yachting magazine's method: remove the cowl and all the plugs, screw in the gauge, crank for about five seconds so the engine turns at least five times, write down the reading and move to the next cylinder. Two people make it quicker, one at the key and one at the gauge. Many outboards still inject fuel with the plugs out, so no flames or smoking nearby.
Yachting magazine notes that pros differ on warming the engine first and on opening the throttle, and that neither is critical. Write down which way you did it.
Citing it in a report
"Port outboard, compression in psi: 1: 135, 2: 130, 3: 115, 4: 132. Cylinder 3 is 15% below the highest, outside the 10% generally accepted. Recommend a leak-down test and a dealer inspection of cylinder 3 before purchase." Give every reading with its unit. Say whether the engine was warm.