Rolling stock
NMRA Car Weight Chart (RP-20.1)
Quick answer
NMRA RP-20.1 recommends 1 oz plus 0.5 oz per inch of car body length for HO rolling stock, so a 6 in boxcar should weigh about 4.0 oz. N scale uses 0.5 oz plus 0.15 oz per inch; O scale uses 5 oz plus 1 oz per inch. Body length excludes the couplers.
An underweight car derails on curves and turnouts that a correctly weighted one clears without issue, and an overweight car drags down what a locomotive can pull. NMRA Recommended Practice RP-20.1 solves both problems with a simple formula: a base weight, plus a fixed amount for every inch of car body length, measured over the body itself and not over the couplers. The car weight calculator runs this exact formula.
RP-20.1 does not publish a figure for every NMRA scale. Where a scale has no published RP-20.1 figure, this chart says so rather than estimating one, because an invented weight recommendation is worse than no recommendation at all.
What is the RP-20.1 weight formula for each scale?
Recommended weight = base weight + (per-inch weight x car body length in inches). These are the five scales RP-20.1 publishes a figure for.
HO cars should weigh 1 oz plus 0.5 oz for every inch of body length; O scale cars need five times the base weight of HO.
| Scale | Base weight | Per inch of car body length |
|---|---|---|
| N | 0.5 oz | 0.15 oz/in |
| TT | 0.75 oz | 0.375 oz/in |
| HO | 1.0 oz | 0.5 oz/in |
| S | 2.0 oz | 0.5 oz/in |
| O | 5.0 oz | 1.0 oz/in |
RP-20.1 does not publish a base weight and per-inch figure for Z or G scale. No figure is given here for those scales rather than an estimated one.
How much should a common HO freight car actually weigh?
Applying the HO formula (1.0 oz + 0.5 oz per inch of body length) to typical car lengths, in both ounces and grams (1 oz = 28.35 g).
A typical 6 in HO boxcar should weigh about 4.0 oz (113.4 g); a 10 in car should weigh about 6.0 oz (170.1 g).
| Car body length | Recommended weight (oz) | Recommended weight (g) |
|---|---|---|
| 4 in | 3.0 oz | 85.1 g |
| 5 in | 3.5 oz | 99.2 g |
| 6 in | 4.0 oz | 113.4 g |
| 8 in | 5.0 oz | 141.8 g |
| 10 in | 6.0 oz | 170.1 g |
| 12 in | 7.0 oz | 198.5 g |
Why is car body length measured without the couplers?
Couplers add length that has nothing to do with the mass of the car body sitting on the trucks, and including them would recommend more weight than the car's actual bulk needs. RP-20.1 measures over the body, meaning end sill to end sill, not coupler face to coupler face. Measuring a fleet consistently this way, with a HARDELL Digital Caliper 6 Inch with Large LCD or a tape measure against the car body only, is what keeps the formula's output meaningful across a mixed roster of different manufacturers.
What happens if a car is under or over the RP-20.1 weight?
An underweight car is the more common problem: it rides light over turnout frogs and superelevated curves and is disproportionately likely to derail in exactly those spots, especially trailing a heavier car that tugs at the coupler on a curve. An overweight car is safer against derailment but adds directly to the tonnage a locomotive has to pull, which matters most on a grade; see the grade percentage chart for how quickly that tonnage cost adds up on a climb. RP-20.1 weight is a target to converge a fleet toward, not a hard pass-fail test for any single car.
Sources
- NMRA Recommended Practice RP-20.1, weight table for rolling stock
Frequently asked questions
What is NMRA RP-20.1?
RP-20.1 is the NMRA Recommended Practice for rolling stock weight. It gives a base weight plus a per-inch-of-body-length addition for several scales, aimed at keeping a mixed fleet of cars from different manufacturers running at a consistent, derailment-resistant weight rather than whatever weight each car happened to ship at.
Does RP-20.1 cover every NMRA scale?
No. Published figures exist for N, TT, HO, S and O. Z and G scale are not covered by a published RP-20.1 base-and-per-inch figure, so this chart does not provide one for those scales rather than estimate a number the standard itself does not state.
How do I measure a car body length for the formula?
Measure end sill to end sill along the car body itself, not from coupler face to coupler face. Coupler length varies by brand and coupler type and has nothing to do with the mass the trucks are supporting, so including it in the length measurement would overstate the recommended weight.
Is a heavier-than-recommended car ever a problem?
Yes, mainly on a grade or with a long train, where extra tonnage per car adds up across the whole consist and increases the chance of a stall or a string-line on a curve. A car that is moderately over RP-20.1 weight is not a derailment risk by itself, but a whole fleet weighed heavy raises the total load a locomotive has to pull.
Should locomotives be weighed to the RP-20.1 formula too?
RP-20.1 targets rolling stock, cars and cabooses, not locomotives. A locomotive's weight is set by its motor, gearing and the traction it needs to pull a train, and manufacturers generally build locomotives as heavy as their mechanism allows rather than to a lightweight car-style target.
What tool is best for weighing a fleet of cars consistently?
A basic kitchen or postal scale that reads in fractions of an ounce or in grams is accurate enough for RP-20.1 work; the goal is consistency across the fleet, not laboratory precision. Weigh every car the same way, on the same scale, and compare the result to the base-plus-per-inch figure for that car's scale.
Researched, not professional advice. This page is compiled from published manufacturer specifications, instruction manuals, published NMRA standards and recommended practices, and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are hobby convention rather than a standard, and they are labelled that way wherever they appear. Always check a radius, a clearance or a grade against your own equipment before you cut wood or lay rail, because manufacturers vary and so does what your specific models will tolerate. Any permanent mains wiring in a layout room is work for a licensed electrician to your local code.