Trailer Axle Placement Calculator
Where the axle goes so the tongue carries the share it should — from the load, not from a rule of thumb.
Pre-filled with the worked example below — a 20 ft deck loaded evenly. Change every field to your own trailer.
Verify before you tow. These figures are a planning aid, not professional or engineering advice. Your real limits are the ones printed on your vehicle's door-jamb placard, your trailer's VIN plate, and your owner's manual - and those override anything calculated here. When numbers are close, weigh the rig and go with the more conservative answer.
The formula, and where it comes from
A loaded trailer rests on two supports: the coupler and the axles. Take moments about the axle centre and the whole thing falls out in one line.
Let W = loaded trailer weight
L = coupler to centre of gravity
A = coupler to axle centre
T = tongue weight
Moments about the axle: T x A = W x (A - L)
T A - L L
Tongue weight fraction f = - = ----- = 1 - -
W A A
L
Rearranged for a target f: A = -----
1 - f Two things follow immediately. The axle always sits behind the centre of gravity, by exactly f × A. And the trailer's own length never appears — only where the load sits. A 20 ft trailer and a 16 ft trailer loaded to the same centre of gravity want the axle in the same place.
Worked example
20 ft deck, 4 ft tongue, load spread evenly,
5,000 lb loaded, target 12% on the tongue
CG from the coupler L = 4 + 20/2 = 14.00 ft
Axle from the coupler A = 14 / (1 - 0.12) = 15.91 ft
Axle behind the CG = 15.91 - 14.00 = 1.91 ft (22.9 in)
Axle from the deck front = 15.91 - 4 = 11.91 ft
Check: T = 5,000 x (15.91 - 14) / 15.91 = 600 lb
600 / 5,000 = 12.0% correct The 60% rule, derived
Builders quote "put the axle about 60% of the way back along the deck". Run the formula for an evenly loaded deck and that is exactly what comes out:
| Deck | Tongue | Target tongue weight | Axle, from deck front | As % of deck |
|---|---|---|---|---|
| 20 ft | 2 ft | 12% | 11.64 ft | 58.2% |
| 20 ft | 4 ft | 12% | 11.91 ft | 59.5% |
| 20 ft | 6 ft | 12% | 12.18 ft | 60.9% |
| 20 ft | 4 ft | 10% | 11.56 ft | 57.8% |
| 20 ft | 4 ft | 15% | 12.47 ft | 62.4% |
Across the whole safe range of tongue weights and a sensible span of tongue lengths, the answer sits between 58% and 62%. The rule of thumb is a real result.
It holds only while the load is spread evenly. The rule is really a statement about where the centre of gravity ends up on a uniformly loaded deck. Put a pallet of blocks at one end and the CG moves — and the axle has to move with it, or the tongue weight goes with it instead.
How much an inch is worth
Differentiating the tongue weight with respect to axle position gives W × L / A² — the pounds of tongue weight gained per foot the axle moves back. On the worked example that is 5,000 × 14 / 15.91² = 277 lb per foot, or about 23 lb per inch.
Two inches of build error is nearly 50 lb of tongue weight. On a trailer sitting near the edge of the 10–15% band, that is the difference between a rig that tracks and one that does not.
Common questions
Where should the axle go on a trailer?
Behind the centre of gravity, by enough that the tongue carries 10% to 15% of the loaded weight. The exact distance is set by where the load actually sits, not by the length of the trailer.
The formula is short: axle distance from the coupler = CG distance from the coupler divided by (1 minus the tongue weight fraction). At 12% tongue weight and a CG 14 feet back, the axle centre goes at 14 / 0.88 = 15.91 feet.
Is the "60% of the deck" rule wrong?
No — it is a good approximation, and you can derive it. For a deck loaded evenly, the CG sits at the middle of the deck. Put that through the formula at 10% to 15% tongue weight and the axle lands between roughly 58% and 62% of the way back along the deck, depending on tongue length.
So the rule of thumb is a real result, not folklore. It stops being safe the moment the load is not spread evenly — a single heavy item well forward or well back moves the CG, and the axle has to move with it.
How much does an inch of axle position matter?
More than most people expect. The sensitivity is the trailer weight times the CG distance, divided by the axle distance squared.
On a 5,000 lb trailer with a 14 ft CG and the axle at 15.9 ft, that works out at about 277 lb of tongue weight per foot — roughly 23 lb per inch. Getting the axle two inches out of place changes tongue weight by nearly 50 lb, which is enough to move a marginal setup out of the safe band.
Where does the axle go on a tandem?
The calculation places the CENTRE of the pair, then the two axles sit either side of it, half the spread each way. A common spread is 33 to 35 inches.
That assumes the two axles share the load equally, which is what an equalised leaf-spring suspension is for. On level ground with a working equaliser it is close enough to true for this purpose.
What if the answer puts the axle behind the back of the deck?
Then the load is too far back to reach your target tongue weight, and no axle position fixes it. The calculator says so rather than returning a number you cannot build.
The fix is to move weight forward — not to shove the axle rearward until the arithmetic works, which is how you end up with a trailer that sways.
Related tools
- Tongue Weight CalculatorFind your target tongue weight and check a measured value against the safe range.
- Trailer Axle Weight CalculatorSplit trailer weight across axles and check each one against its rating.
- Trailer Axle Weight ChartCommon axle capacities, what a bare axle weighs, and how to identify the axle you have.
Sources
- SAE J2807: Performance Requirements for Determining Tow-Vehicle Gross Combination Weight Rating and Trailer Weight Rating - the 10% tongue weight assumption behind published tow ratings
- NHTSA - Towing a Trailer: Being Equipped for Safety - tongue weight and trailer stability
- Tongue Weight Calculator - check a measured tongue weight against the safe range
- Trailer Axle Weight Calculator - load per axle once the axle is placed