Ratchet Strap Length Estimator
Measure your load's height and width and the distance between the two anchor points, choose over-the-top or direct routing, and pick how many wraps you want on the ratchet mandrel. The estimator works out how much webbing the path needs, adds allowances for the ratchet body, hooks, mandrel wraps and hand-pull slack, then rounds up to the nearest common strap length (8, 10, 12, 15, 16, 20 or 27 ft) and suggests how the fixed and adjustable ends should split. It is for anyone buying straps for a specific trailer, truck bed, roof rack or moving job. Lengths are estimates from stated assumptions; measure before you buy. Everything runs in your browser; nothing is uploaded, stored or sent.
Assumptions you can change
These are assumptions, not measured specs: hardware and fixed-end lengths vary by maker and width. Check the listing for your strap.
Calculating...
How it works, and what it does not cover
Path length. For an over-the-top strap the tool assumes both anchors sit at deck level and the load is a box. If the anchors are wider than the load, each side of the strap runs on a slope from the anchor to the top edge: side run = square root(gap squared + height squared), where gap = (anchor spacing - load width) / 2. The path is two side runs plus the width across the top. If the anchors are narrower than the load, the strap goes straight up each side, so the path is two heights plus the width. For a direct tiedown the path is one diagonal from the anchor to the hard point: square root(anchor distance squared + height squared); enter the horizontal distance from the anchor to the hard point in the anchor spacing field and the hard point's height in the height field.
Allowances. Webbing needed = path + hardware allowance + mandrel wraps + hand-pull slack. The hardware allowance (default 1.5 ft) covers the ratchet body and the two hooks, which take up length but are not webbing you can route. Mandrel wraps default to 2, at an assumed 3 inches of webbing per wrap, which matches the WSTDA and strap-maker guidance of two to three wraps before tensioning. Hand-pull slack (default 1 ft) is the webbing you pull through by hand so the ratchet only has to take up the last foot or so; a ratchet that must take up several feet overfills its mandrel.
Rounding and the two ends. The needed length is rounded up to the next common advertised length: 8, 10, 12, 15, 16, 20 or 27 ft. The fixed end suggestion keeps the ratchet on the sloped side run below the top edge, so it does not sit on a corner or on top of the load, and the adjustable end is the remainder. If the rounded-up size leaves a lot of spare webbing, the tool says so; tie off the tail so it cannot flap or feed back into the ratchet.
What it does not cover
- Irregular loads (motorcycles, kayaks, machinery) where the strap wraps around handlebars, soft loops or frames. Measure the real path with a rope and use that as the width.
- Edge protectors, corner guards and twists that add an inch or two each; add them to the slack if you use several.
- Strap strength. Length says nothing about rating. Use the tie-down WLL calculator to check working load limit and count, and the strap angle calculator to check a flat routing.
- Retractable straps, whose advertised length is the full extended webbing with no separate fixed end.
Estimate from stated assumptions, not a spec; measure your load and read the listing before you buy. The label on your strap and the current regulation govern how the strap is used. No products are recommended inside this tool; see the disclosure page for the site's relationships.
Data last verified 2026-10-28. Prices, hours and rules change; results are estimates from the formulas shown; confirm the working load limit on your strap label and the current regulation before you haul.
Sources
- Web Sling & Tie Down Association (WSTDA), T-1 synthetic web tiedown standard (2 to 3 mandrel wraps guidance)
- US Cargo Control, how to use a ratchet strap (manufacturer threading and wrap guidance)
- Rhino USA, how to use ratchet straps (manufacturer guidance)
- 49 CFR 393.104 (tiedowns must be in proper working order; edge protection)
Related guides
Frequently asked questions
Is the length on the package the whole strap?
Usually the advertised length (15 ft, 20 ft, 27 ft) is the long, adjustable end measured from the hook to the free tail. The short fixed end on the ratchet adds roughly 1 to 1.5 ft on most consumer straps, but makers differ, so read the listing. This estimator compares your need against the advertised length and treats the hardware as a separate allowance you can edit.
How many wraps should be on the ratchet mandrel?
Strap makers and the WSTDA T-1 guidance describe two to three wraps of webbing on the mandrel before you start tensioning. Fewer than two and the webbing can slip or the ratchet may not hold; more than three or four and the mandrel overfills and jams. The estimator adds webbing for the number of wraps you pick, at an assumed 3 inches per wrap.
Why does it suggest a strap that is longer than my measurement?
The strap has to reach, wrap the mandrel two or three times, and still leave slack you can pull by hand before ratcheting. Those three allowances are added to the measured path, then the result is rounded up to the next common length. Too short is a failed tie-down; too long is just tail to tie off.
What if the nearest common length is far longer than I need?
Extra webbing is fine as long as you tie off the tail so it cannot flap or feed into the ratchet. The tool flags cases where a strap would leave several feet of spare webbing, which is where a shorter size or a retractable strap would be tidier.
Does this tell me how strong the strap should be?
No. Length and rating are separate choices. Use the tie-down WLL calculator on this site to check how many straps and what working load limit the load needs, then size the length here.