TowingMath

How to Weigh a Trailer at a CAT Scale

Two weighings, about fifteen minutes and roughly twenty dollars gives you every number the calculators on this site ask for - measured rather than estimated.

What you get

A CAT scale has three platforms, and a hitched rig gives you a reading from each:

Add a second weighing with the trailer unhitched and you can isolate tongue weight, which unlocks the payload calculation, the sway question and the axle margins all at once.

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.

Step by step

1. Load exactly as you travel

Full fuel, water and propane if that is how you tow. Passengers aboard. Cargo where it normally rides. Hitch set up as normal with the weight distribution bars tensioned.

A rig weighed half-loaded produces numbers for a vehicle you never drive. The whole value of weighing is that it replaces assumptions with the actual configuration.

2. First weighing, hitched

Pull onto the scale so each axle group sits squarely on its own platform - steer on the first, drive on the second, trailer axles on the third. Nothing should bridge two platforms. Press the call button, give the attendant your details, and stay put until told to pull forward.

Record all three numbers, plus the total.

3. Second weighing, unhitched

Park and drop the trailer somewhere safe in the lot. Return to the scale with the tow vehicle alone and ask for a re-weigh - same day, same location, and it costs a fraction of the first pass.

Record the steer and drive axle weights again.

4. Do the arithmetic

Weighing 1 - hitched
  Steer axle                    3,140 lb
  Drive axle                    3,940 lb
  Trailer axles                 6,090 lb
  Total                        13,170 lb

Weighing 2 - tow vehicle alone
  Steer axle                    3,200 lb
  Drive axle                    3,100 lb
  Tow vehicle total             6,300 lb

TONGUE WEIGHT
  = hitched tow vehicle - unhitched tow vehicle
  = (3,140 + 3,940) - 6,300
  = 7,080 - 6,300
  = 780 lb

LOADED TRAILER WEIGHT
  = trailer axles + tongue weight
  = 6,090 + 780
  = 6,870 lb

TONGUE WEIGHT PERCENTAGE
  = 780 / 6,870 x 100
  = 11.4%   (inside the 10-15% range)

FRONT AXLE CHECK
  Unhitched steer  3,200 lb
  Hitched steer    3,140 lb
  Still carrying 98% of its unhitched load - the
  weight distribution hitch is doing its job.

What to do with each number

NumberCheck it againstTool
Tongue weight 10-15% of loaded trailer weight; your payload Tongue weight
Loaded trailer weight Trailer GVWR GVWR margin
Trailer axle weight GAWR per axle, and your tire ratings Axle weight
Drive axle weight Rear GAWR on the door placard Payload
Steer axle, both passes How much front load the WD hitch restores WD hitch
Combined total GCWR Towing capacity

Common mistakes

Common questions

How much does it cost to weigh at a CAT scale?

A first weighing is typically in the low-to-mid teens of dollars, with re-weighs on the same day at the same location costing a few dollars. Prices vary by location and change over time, so check the CAT Scale app or the sign at the scale house.

For the two-weighing method here, the second pass usually qualifies as a re-weigh, which is the cheap one.

Do I need to empty my tanks before weighing?

No - weigh the rig exactly as you travel. That is the entire point. If you tow with a full fresh tank, weigh with a full fresh tank.

Weighing an empty rig produces numbers that describe a vehicle you never actually drive, which is worse than not weighing at all because it feels like data.

Can I weigh without unhitching?

You can get most of what you need in one weighing: trailer axle weight comes straight off the third platform, and the tow vehicle's steer and drive axles come off the first two.

But tongue weight requires the difference between hitched and unhitched tow vehicle weight, so isolating it needs the second pass. Since tongue weight drives the payload calculation and the sway question, it is worth the extra few dollars.

What if the scale platforms do not line up with my axles?

Reposition until each axle group sits squarely on its own platform, with nothing bridging a gap. A tandem trailer axle pair counts as one group and belongs entirely on one platform.

If the rig is long enough that it will not fit the layout, tell the attendant. Truck stop scales handle tractor-trailers routinely and the staff deal with awkward configurations constantly.

Related tools

Sources