Tandem Slide Calculator

Scale ticket in — holes to slide out. Real lever math from your trailer's actual geometry, not a one-size-fits-all "400 pounds per hole" guess.

Exact beam formula, assumptions shown Your weights never leave this device Free — no signup

From your scale ticket

Legal targets: steers per your steer rating, drives ≤ 34,000, tandems ≤ 34,000, gross ≤ 80,000 (federal interstate limits).

Before you slide: many states limit how far back your tandems can be (kingpin-to-tandem laws — California's 40-ft KPRA is the famous one) and rearward slides can also violate length/overhang rules. Check your route's rules before sliding rear. And always re-weigh — this is a prediction from a simplified beam model, and where the tractor's share lands between steers and drives depends on your tractor's geometry.
Model: trailer treated as a beam between kingpin and tandem center — Tnew = T × d ÷ (d ± n·s). Federal limits: 23 CFR 658.17 / FHWA. Reviewed July 2026

The actual math (so you can sanity-check it)

Your trailer is a beam resting on two supports: the kingpin (carried by the tractor) and the tandem center. The freight's center of gravity doesn't move when you slide — the support does. Move the tandems rearward and the lever arm gets longer, so the tandems carry less and the tractor carries more. Move them forward and the opposite happens.

If your tandems currently scale T pounds at distance d from the kingpin, sliding n holes of spacing s rearward gives a new tandem weight of T × d ÷ (d + n·s). That's why the per-hole change isn't one fixed number: it depends on your load, your trailer, and how far you've already slid.

Worked example: tandems 35,200 (1,200 over), drives 32,400, kingpin-to-tandems 36 ft, 6-inch holes. Three holes rearward: 35,200 × 36 ÷ 37.5 = 33,792 lb on the tandems — 1,408 lb moved to the tractor. If nearly all of it lands on the drives, they'd sit around 33,800: legal, but tight. Re-weigh.

What about the fifth wheel?

Sliding the fifth wheel moves weight between steers and drives using the same lever idea on the tractor's wheelbase — forward loads the steers, rearward loads the drives. We don't predict exact steer/drive numbers here because that split depends on tractor geometry this tool doesn't ask for; use small moves and the scale.

Why apps say "250–500 lb per hole"

Because that's what the formula above works out to across common trailers: 4-inch holes at long kingpin distances sit near the bottom of that range, 6-inch holes with heavy tandems near the top — and with tandems slid far forward (a short kingpin distance), the first hole can move more than 500 lb. The flat rule gets you close; the formula plus a re-weigh gets you legal.