BoardcallHow the number is worked out
Start with body weight in kilograms and multiply by a coefficient for ability. A beginner needs close to their own weight in litres; an expert needs about a third of it. That single step is most of the answer:
| Skill | Multiplier | 75 kg rider |
|---|---|---|
| Beginner | 0.95 | 71 L |
| Intermediate | 0.60 | 45 L |
| Advanced | 0.42 | 32 L |
| Expert | 0.35 | 26 L |
Then the day moves it. Short-period wind swell has very little energy in it, so you need more foam just to get into the wave — up to 12% more under nine seconds. Long-period groundswell does the work for you, so you can drop about 10%.
The part almost everyone gets backwards
Big waves need more volume, not less. It feels wrong, because the boards look thinner. But you need paddle power to get into an overhead wave early, and being late is how people get hurt. That is exactly why a big-wave gun is longer and more voluminous than the shortboard the same person rides on a Tuesday — not smaller.
Volume only comes down through the middle of the range, roughly three to six feet, where the wave has enough push to carry you and a thinner rail starts to earn its keep.
Estimating volume from dimensions
Length × width × thickness in inches, times 0.0163871 to get litres, times a shape factor for the outline — a longboard fills its box far more completely than a gun does. The estimate usually lands within a litre or two of a stamped figure. A 6'0" × 19½" × 2½" shortboard comes out at about 28.8 L, which is what that board really is.
What this calculator can't do
It doesn't know what the waves are doing at your break this morning — you have to tell it. That is the honest limit of any calculator on a web page, and it is the reason the app exists: Boardcall reads the swell, wind and tide at your break, works out the face height itself, and then runs this same arithmetic against the boards you actually own — so instead of a number, you get a board.