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About this tool
Check dynamic roof load limits against rack plus cargo weight, with payload, rollover stability and fuel penalty.
This checker works out how much cargo a roof can legally and safely take by subtracting the rack and any carrier from the lower of two ratings: the vehicle's dynamic roof load from the handbook, and the rack system's own rated load. It then checks that the roof load still fits inside gross vehicle mass, since roof weight is part of payload rather than extra to it, and shows what the load does to rollover resistance using NHTSA's Static Stability Factor, track width divided by twice the centre of gravity height. A fuel figure is included from the drag equation, because an added frontal area costs half the air density times CdA times speed squared in extra force for every kilometre driven.
Open Roof Rack Load Checker on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
The vehicle limit and the rack limit are checked together, and the lower one is used — the mistake most people make is using only one.
The bars and the box come off the allowance before any cargo does, which is what the handbook figure actually means.
The load's effect on the centre of gravity is shown as a Static Stability Factor and an NHTSA rollover band, not as vague advice to drive carefully.
Yes. The dynamic roof load in the handbook is the total mass on the roof, so the bars, the feet, a roof box and a bike carrier all come out of it before any luggage does. A 75 kg limit with 6 kg of bars and a 20 kg box leaves 49 kg for cargo.
Dynamic roof load is the maximum while the vehicle is moving and is the figure that matters for driving; static roof load applies only when parked and is what a roof tent's occupants are assessed against. The static figure is much higher, and it must come from the manufacturer rather than be assumed as a multiple of the dynamic one.
At 100 km/h a large roof box adding roughly 0.30 m² of drag area costs about 1.7 litres per 100 km on a petrol car, and even bare bars adding 0.08 m² cost around 0.4 litres. Because drag rises with the square of speed, the same box costs substantially more at 120 km/h than at 90.
It raises the centre of gravity, which lowers rollover resistance measurably: 66 kg on the roof of a typical car can move the Static Stability Factor from about 1.29 to 1.21, a whole NHTSA rollover band. It is manageable within the stated limits, but reduce speed, allow more distance for braking, and be careful with crosswinds and sudden lane changes. Follow the vehicle and rack manufacturers' instructions rather than this estimate.
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<p style="font-size:12px;margin:4px 0 0">Widget by <a href="https://www.altftool.com/tools/all/roof-rack-load-checker?utm_source=embed&utm_medium=widget">AltFTool — free online tools</a></p>