Every brand's volume calculator quietly steers you toward the boards that brand makes. SurfCalc pools twenty published calculators, shaper interviews and surf-school guides into one neutral model. This page shows the working: what each source would tell the surfer below, the constants, and where the brand tools bend.
Shortboard volume for the inputs above, using each source's own published rule. Bars show stated ranges; the tick is the midpoint. Brand tools bunch tightly at the expert end and scatter at the beginner end.
Every source expresses its advice, directly or after conversion, as litres of board per kilogram of surfer. Pooling 20 sources gives a clear ladder. The expert end is settled: nine WCT-level riders with published board volumes sit between 0.333 and 0.367 L/kg (Slater 0.33, Fanning 0.35, Andino 0.35, Reynolds 0.36, Gilmore 0.37), and Firewire, Lost, JS, NSP and Rusty all publish 0.35–0.36.
The beginner end is where the brand bias is. Shortboard makers top out around 0.55–0.70 L/kg because that is the most foam their outlines carry. Surf schools, softboard makers and board-agnostic guides (Surf Simply, Surfex, Rapture, Catch Surf dealer advice) say 0.8–1.25. The model uses the shortboard-basis number as the base and recovers the rest through the board-family multiplier, so a beginner on a soft-top lands at roughly 1.1 L/kg, where the schools put them.
| Tier | Sources | Median | Spread | Model base |
|---|---|---|---|---|
| 5 Expert / pro | 12 | 0.350 | 0.30–0.38 | 0.35 |
| 4 Advanced | 19 | 0.375 | 0.31–0.53 | 0.38 |
| 3 Intermediate | 17 | 0.465 | 0.39–0.68 | 0.47 |
| 2 Beginner / int. | 7 | 0.525 | 0.45–0.90 | 0.58 |
| 1 Beginner | 17 | 0.700 | 0.45–1.25 | 0.72 |
L/kg, shortboard basis. Medians of each source's midpoint. Beginner shortboard-only median is 0.575; all-source 0.70.
The same surfer rides more foam on a longer, flatter board, and the gap grows with skill: an expert's longboard is 2.2× their shortboard (Harley Ingleby, 85 kg, 65 L vs a WCT shortboard at 30 L), while a beginner's soft-top is only about 1.5× the "beginner shortboard" number because that number is already inflated. Multipliers are anchored on NSP's code (1.0 / 1.1 / 1.3 / 1.9), the Swaylocks shaper rule (0.31 / 0.47 / 0.62 / 0.78 L/kg for fit surfers), SurfSizer's 78 kg comparison (30 / 32 / 45 / 70 L) and softboard makers' weight ratings (0.7–1.2 L/kg).
| Family | T1 | T2 | T3 | T4 | T5 |
|---|---|---|---|---|---|
| Performance shortboard | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
| Fish / hybrid / groveler | 1.05 | 1.07 | 1.10 | 1.12 | 1.12 |
| Midlength / funboard | 1.25 | 1.32 | 1.40 | 1.55 | 1.60 |
| Longboard | 1.45 | 1.55 | 1.75 | 2.00 | 2.20 |
| Soft-top | 1.50 | 1.55 | 1.60 | 1.65 | 1.70 |
Applied as multipliers to the ratio, so they scale with the rider. Values sit between the timid (JS: age ±1 %, fitness ±2.5 %) and the aggressive (Surfboard Empire: 51–60 ×1.2, 61+ ×1.3; poor fitness ×1.2), weighted toward sources that quote litres.
| Factor | Model | Evidence |
|---|---|---|
| Age | 30s +2 % · 40s +5 % · 50s +10 % · 60+ +15 % | Guild Factor "older experienced" band +5–10 %; Empire +8/+20/+30 %; Swellnet threads, 47–54 yr riders at 0.40–0.47 L/kg |
| Fitness | very fit −4 % · below avg +6 % | GF super-fit vs normal band ≈5 %; Empire 1.0/1.1/1.2; Breakfinder −1 / +1.5 L |
| Frequency | daily −3 % · monthly +4 % · rarely +8 % | JS 0.90 → 0.95; Rusty "< 3 days a week raises it"; Surfex "twice a month: treat chart as minimum" |
| Wave power | weak +6 % · hollow −4 % | TotalCalcHub +3 L / −2 L; Breakfinder ±2 L; Cory Russell groveler +9 % |
| Wave size | < 2 ft +5 % · 6–8 ft +5 % · 8 ft+ +12 % | Cory Russell step-up +5 %, semi-gun +15 %; JS valley shape (least at 2–4 ft) |
| Wetsuit | spring +1.5 % · 3/2 +4 % · 4/3+ +6 % | Southern Man +0.8 / +1.5 / +2.2 L; JS +1 / +3 / +4 %; shapers "+1 L cold-water" |
| EPS / epoxy | −4 % | Southern Man −1.5 L; Rider Shack −0.5–1 L; SurferToday −2–3 L |
| Fresh water | +3 % | Sea water 1.025–1.027 kg/L vs 1.000: 2.7 % less buoyancy |
Weight × 0.4 L/kg, then every other factor is a fixed litre offset anchored to a 70 kg reference. Beginner tops out at 0.58 L/kg and the whole input space at 70 kg spans only 25–45 L: the calculator can only recommend boards JS makes. Wave size moves the answer by 2 L and fitness by 1.3 L; skill and frequency do almost all the work. Code comments say the skill factors were "recalibrated against real pro rider data".
A lookup table that is exactly Advanced × 1.00 / 1.05 / 1.25 / 1.50 / 2.00, with Advanced = 0.350 L/kg from 64–98 kg, drifting up to 0.44 for very light riders. Two inputs only; the "better waves ↔ weak waves" slider has no published values.
A nomogram, not a formula: you pick your own band (0.34–0.50). Honest about conditions and wetsuits pushing you up the band, silent on how far. The pro data points on the page are the best calibration anchor in the industry.
The only brand tool with a board-family input (×1.0 / 1.1 / 1.3 / 1.9). Four ability tiers, no age or conditions; height is collected and ignored.
A weight × ability table multiplied by age (up to ×1.3) and fitness (up to ×1.2), then scaled by a flat 0.75 "offset". Its base table is the most generous in the set; the offset pulls it back below everyone else at the advanced end (0.31 L/kg).
Board-agnostic and explicit that its number is the smallest practical board, not the recommendation. Its four-level ladder (1.0–1.5 / 0.8–1.0 / 0.5–0.8 / 0.4–0.5 / 0.35–0.40) is what a beginner-to-intermediate ladder looks like when nobody is selling shortboards.
Numeric age increments (Rusty: "no specific increments"), a height term with evidence behind it, and any validation against outcomes. Treat every number here, including this model's, as a starting point to demo against.