Rig build-out payload & GVWR calculator

Every part of a build-out adds weight: the battery bank and wiring you sized with the other tools, drawers, a roof rack and tent, water, people, gear and trailer tongue weight all come out of the same payload number. Check it before you buy and bolt on.

Start from

Placard: occupants + cargo (lb)"should never exceed XXX lb"

Weight added since the factory (lb)battery bank, wiring, drawers, bumpers, winch…

GVWR (lb)door-jamb certification label

Weighed weight, empty of people/cargo (lb)fuel full, mods installed

People (lb)

Gear and cargo (lb)

Roof rack (lb)

Rooftop tent (lb)

Other roof gear (lb)

Water carried (gallons)× 8.34 lb/gal

Loaded trailer weight (lb)

Tongue weight %10–15% typical

Measured tongue weight (lb)overrides the %

Roof rack / roof dynamic rating (lb)lowest of vehicle roof, rack, crossbars

Hitch max tongue weight (lb)

Enter values above.

The spreadsheet adds: in the bundle's Payload sheet: a 150-row gear list with categories and locations, front/rear axle split from CAT-scale weights, parked roof and tent load, tow/hitch/GCWR checks, the heaviest 5 items, and lb or kg. Free vs. spreadsheet

Safety: this is a planning estimate, not an engineering sign-off. DC systems can start fires and lithium batteries can deliver very high fault current. Check every result against the manufacturer's instructions and current ABYC/NEC or local rules, put a correctly rated fuse or breaker close to the battery on every positive cable, and have a qualified installer check work you are not sure about. We are not responsible for how results are used.

How this is calculated

Placard method. The yellow tire placard on the driver's door jamb (required by FMVSS 110) states "the combined weight of occupants and cargo should never exceed XXX lb". NHTSA has explained that equipment added after the factory reduces that capacity, so subtract everything bolted on since: bumpers, winch, sliders, drawers, skid plates.

Scale method (better). Weigh the rig at a CAT or public scale with a full tank and all permanent mods but no people or loose gear. Payload left = GVWR (on the certification label) − weighed weight. This catches the weight you forgot about.

What counts. Payload used = people + gear + roof rack + tent + roof gear + water (8.34 lb per U.S. gallon) + trailer tongue weight. Tongue weight is typically 10–15% of the loaded trailer (U-Haul, vehicle towing guides); a measured value is better. Also stay within each axle's GAWR and the hitch's own tongue rating.

Roof. Roof loads are limited by the dynamic (moving) rating of the weakest part: the vehicle roof, the rack, or the crossbars. Static (parked) ratings are higher because they spread the tent's occupants over the frame. Some makers quote about three times the dynamic rating as a guide, but only the manufacturer's figure counts.

Planning a whole build?

The paid spreadsheets run these same tested formulas for a whole build in one saved file. Compare free vs. spreadsheet.

Overland Rig Planning Bundle (12V + Payload + Trip spreadsheets) on Etsy, $18.99

Sources

eCFR 49 CFR 571.110 (FMVSS 110): tire and loading information placard. www.ecfr.gov
NHTSA interpretation GF004408: placard 'combined weight of occupants and cargo'; added equipment reduces it. www.nhtsa.gov
NHTSA interpretation 08-003469drn-rev: GVWR vs unloaded weight plus capacity. www.nhtsa.gov
U-Haul: Hitch selection guide (tongue weight about 10–15% of trailer weight). www.uhaul.com
Nissan 2026 towing guide (tongue weight counts against payload). www.nissanusa.com
Thule: roof-top tent load guidelines (dynamic vs static load). www.thule.com
Prorack: load ratings explained (dynamic, static, off-road). www.prorack.com.au
USGS: A million gallons of water (8.34 lb per U.S. gallon). www.usgs.gov