How to Calculate Caster Load Rating for Your Cart

Buying casters by eyeballing the load is how carts end up with flat-spotted wheels, seized swivels, and a bent top plate six months in. The math is simple, but almost nobody does it correctly — and the part most buyers miss is that you should never divide the load by four.

Here is the calculation shops actually use, plus how to read it against a real spec sheet.

The three-wheel rule

A four-caster cart does not distribute weight evenly across four wheels. Floors are not flat. Cracks, expansion joints, dock plates, and drain slopes mean that at any given moment one caster is carrying little or nothing while the other three carry the whole load. Loads also shift toward one end when a cart is pushed, stopped, or loaded from one side.

So the standard industry approach is to size for three wheels, not four:

Required capacity per caster = (Cart weight + Maximum load) ÷ 3

Round up to the next available rating. Never round down.

Worked example

A steel platform cart weighs 180 lbs empty and will haul up to 2,000 lbs of stock.

• Total = 180 + 2,000 = 2,180 lbs
• 2,180 ÷ 3 = 727 lbs per caster

You need casters rated at 727 lbs or more, each. A 660 lb caster fails this test even though 4 × 660 = 2,640 lbs looks like plenty on paper. Step up to a 900 or 1,000 lb caster instead.

Capacity by wheel size: a real example

Bigger wheels carry more, roll easier, and clear obstacles better. Here is how capacity scales across one line — our Series 40 phenolic caster, which uses the same 2" tread width in every size:

Wheel Size	Capacity (each)	Overall Height	Safe Cart Load (×3)
4" x 2"	900 lbs	5-5/8"	2,700 lbs
5" x 2"	1,000 lbs	6-1/2"	3,000 lbs
6" x 2"	1,000 lbs	7-1/2"	3,000 lbs
8" x 2"	1,000 lbs	9-1/2"	3,000 lbs

Notice that capacity plateaus at 1,000 lbs once you pass 5". Above that point you are buying wheel diameter for rollability and obstacle clearance, not for more capacity. If you need more than 1,000 lbs per position, you move up to a heavier series — not just a bigger wheel in the same series.

Four things that change the number

• Floor condition. Broken concrete, thresholds, and dock plates add shock loading. Rough floors and frequent obstacles are the classic case for adding margin on top of the ÷3 figure.
• Speed and towing. Casters are rated for manual walking speed. Tow-line or powered applications generate impact loads well beyond a static rating — check with us before speccing those.
• Load placement. A tall or one-sided load shifts weight toward two casters, not three. Concentrated point loads deserve extra margin.
• Duty cycle. A cart moved twice a shift and a cart moved constantly are not the same job, even at identical weight.

Rating is not the only spec that has to fit

A caster that clears the capacity math can still be the wrong part. Before you order, confirm:

• Top plate and bolt pattern. The Series 40 uses a 3-15/16" x 4-1/2" plate with bolt hole spacing of 2-5/8" x 3-5/8" to 3" x 3" and 3/8" mounting bolts. Measure your existing frame before assuming a swap fits.
• Overall height. Mixing sizes on one cart makes it rock. All four positions need the same overall height.
• Wheel material. Phenolic is hard, low rolling resistance, and resists oils, greases and moderate heat — strong for oven lines and heavy carts, but it transmits more noise and floor shock than a softer tread.
• Braking. A top lock brake holds the wheel; a total lock brake holds the wheel and the swivel, which is what you want on a cart that must not creep or rotate.

Build the set correctly

The common configuration for a maneuverable cart is two rigid casters at one end and two swivels at the other. For a cart that must move sideways in tight aisles, use four swivels — and add brakes, because four swivels will wander. Every Series 40 size is offered in Rigid, Swivel, Swivel with Brake, and Swivel with Total Lock, so you can build a matched set at one height.

Frequently asked questions

Why divide by 3 instead of 4?
Because floors are never perfectly level. On any real surface, three casters can end up carrying the entire load while the fourth is unloaded. Dividing by three builds that condition into the spec.

Do I add the weight of the cart itself?
Yes. Dead weight is still weight. Add the empty cart, any fixtures or shelving mounted to it, and the maximum payload before dividing.

Does a bigger wheel always mean higher capacity?
No. In the Series 40 line, capacity tops out at 1,000 lbs from 5" up. Larger diameters buy easier rolling and better obstacle clearance, not more rating.

Can I mix wheel sizes on one cart?
Not on the same cart. Different overall heights cause rocking, uneven load transfer, and premature wheel failure. Keep all four positions at one height.

What if my calculated number falls between two ratings?
Always go up. Casters are the cheapest part of a cart to over-spec and the most expensive to replace after a failure.

Load ratings are for manual, walking-speed use on smooth, level floors. Powered, towed, or high-speed applications, shock loading, and sloped or damaged floors all require additional capacity margin. Never exceed the rated capacity of the caster or the structural capacity of the cart frame.