Methodology & sources
Every formula, default, and limitation of these calculators, in the open. If you find an error, email us at support@whetstonetools.com.
The three checks
| Check | Formula | Compared against |
|---|---|---|
| Payload | (trailer weight × pin/tongue %) + hitch hardware + people + cargo | Door-sticker payload ("occupants and cargo" number) |
| Tow rating | Trailer weight (loaded) | Fifth-wheel/gooseneck or receiver tow rating for your configuration |
| GCWR | truck curb (≈ GVWR − sticker payload) + people + cargo + hitch + trailer weight | GCWR from owner's manual / tow guide |
Verdict bands: PASS at ≤90% of a limit, MARGINAL at 90–100%, FAIL over 100%. The overall verdict is the worst individual check, and the limiting factor is named. The 10% margin band exists because every input upstream of a scale is an estimate.
Defaults, and why
- Trailer weight = trailer GVWR. Checking against the loaded limit means the answer stays true on the worst (fullest) day. A dry-weight mode exists for people who want to model a specific load, but GVWR is the default on purpose — naive dry-weight calculators are the #1 source of bad towing advice.
- Pin weight 22% (range 15–28%). Industry planning convention for loaded fifth wheels is 20–25%; 22% is the midpoint used across manufacturer and hitch-maker guidance.
- Tongue weight 12% (range 8–18%). The sway-safe range for conventional trailers is 10–15%. The slider allows excursions outside it so you can see the math, not as a recommendation.
- Hitch hardware 200 lb (fifth wheel) / 100 lb (weight distribution). Typical retail weights; editable.
- Truck curb ≈ GVWR − sticker payload. The Tire and Loading label is computed per-VIN at the factory as GVWR minus the truck as built (full fluids), making this the most accurate free estimate of curb weight available without a scale.
What these calculators deliberately don't do
- No axle-by-axle (RAWR) verdict. Splitting pin weight between axles honestly requires your truck's actual front/rear distribution — scale data. We say so instead of guessing.
- No year/make/model database yet. Charts mislead: two identical-looking trucks can differ by 1,000+ lb of payload depending on options. The door sticker is per-VIN truth and takes 30 seconds to read. (A clearly-labeled estimate mode based on manufacturer tow guides is planned; the sticker will always be the recommended path.)
- No "close enough" passes. Manufacturer ratings are treated as hard limits. Whether to run at 99% of one is between you, your scale, and your judgment — the tool just shows the margin.
Sources
- Tire and Loading / certification label conventions: FMVSS 49 CFR §571.110 (the rule that puts the "occupants and cargo" sentence on your door jamb).
- Manufacturer annual towing guides (Ford, GM, Ram/Stellantis) for GCWR and tow-rating definitions and configuration dependence.
- Pin/tongue percentage conventions: manufacturer and hitch-maker towing guidance (consistent across Ford/GM/Ram guides and major hitch manufacturers' literature).
- Water weight: 8.34 lb/US gallon.
Privacy
The calculators run entirely in your browser. No accounts, no analytics, no cookies, and no inputs ever leave your device. Details: privacy page.
Disclaimer: HitchMath does arithmetic on numbers you provide. It is not safety, legal, mechanical, or purchasing advice, and it cannot account for vehicle condition, modifications, or local law. Verify capacities on your vehicle's labels and owner's manual, and confirm real-world weights on a certified scale before towing.