Fork Oil Level Calculator

Because oil volume is proportional to the cross-sectional area of the tube, and area scales with the square of diameter -- a given percentage change in diameter has roughly twice the proportional effect on volume that the same percentage change in the linear oil-column length (Fork Tube Length minus Oil Level) produces.

No. Oil per Leg, Total Oil Needed, and Air Gap Volume are calculated purely from Fork Tube Inner Diameter, Fork Tube Length, and Oil Level -- Rider Weight only affects the Adjusted Oil Level and Recommended Viscosity suggestions, which are separate tuning recommendations layered on top of the base volume figures.

It steps in 5-10 mm increments: riders over 90 kg get 10 mm less air gap (stiffer), over 80 kg get 5 mm less, under 65 kg get 10 mm more (softer), and under 75 kg get 5 mm more, with no adjustment for riders in the middle of that range -- consistently less air gap (stiffer) for heavier riders and more air gap (softer) for lighter ones.

Only at the extremes, and only in one 5W step: above 95 kg rider weight it suggests one step stiffer than your selected Oil Viscosity, and below 65 kg it suggests one step lighter -- across the broad middle of the rider weight range, Recommended Viscosity simply matches whatever you selected. Because the option list runs 5W to 20W, a rider over 95 kg already on 15W or 20W is left there rather than pushed past 20W, and a rider under 65 kg already on 5W stays on 5W.

Treat them as starting estimates, not fill instructions. The calculator models the fork leg as a completely empty cylinder, subtracting nothing for the damper rod, cartridge, or spring that sit inside the real oil column, so it will read differently from your service manual's published fill volume. Use the manual's volume to fill, bleed the damper by stroking it, and then set the air gap precisely with Oil Level rather than relying on the poured volume alone. Air Gap Volume converts that same linear Oil Level distance into the actual volume of compressible air trapped above the oil -- the figure that more directly relates to how progressively the air spring builds resistance near the end of the fork's travel. Adjusted Oil Level is also capped at the calculator's own 50-200 mm range, and you should cap it again at whatever minimum air gap your service manual specifies -- reducing the air gap past that point stops adding progression and starts risking a hydraulic lock at full travel.