Recommended Car Weight Calculator
Unofficial estimate: no British national body publishes a wagon-weight standard. These figures are this calculator's own estimate, interpolated from MOROP NEM 302's own published figures by scale ratio (Gauge 1 and G are the exceptions — their ratios match NEM's own I and II exactly, so those two aren't estimates) — see Sources below for the exact derivation. Treat it as a starting point, not an authority.
Measure the car body itself — not over couplers.
Measure the full wagon including buffer plates — not the body alone.
Enter this to see how much weight to add or remove to reach the target weight.
Description
Properly weighted rolling stock tracks through curves and rough spots more reliably — too light, and a car risks being pulled off the rails in sharp curves or by a heavier train; too heavy adds drag without improving performance. Two real published standards give target weights for this: the NMRA's RP-20.1 (US) and MOROP's NEM 302 (European). Pick your standards system above.
NMRA RP-20.1 gives a starting weight for the scale plus an additional weight for every actual inch of the car's body length (not counting couplers) — longer cars need proportionally more weight, since the derailing forces in a curve are greater for a longer car.
MOROP NEM 302 gives a minimum mass per millimeter of wagon length, measured over the buffers — a different length basis than RP-20.1's body length, so don't reuse a body-length measurement here. NEM 302 also says mass can be increased above that minimum (by ballast, ideally removable) but shouldn't exceed it by more than 30% without accounting for it when making up a train — this calculator shows that upper figure too.
UK has no official standard at all, so these scale options are this calculator's own estimates rather than citations — see Sources below for exactly how each is derived, and treat them accordingly.
Enter your car's current weight (optional) and this calculator also tells you how much weight to add or remove to reach the target figure.
Sources
NMRA figures are reproduced directly from RP-20.1 "Car Weight" (revised January 1990), covering its eight published scale/gauge combinations (O, On3, S, Sn3, HO, HOn3, TT, N). NEM figures are reproduced directly from MOROP NEM 302 "Wagon Mass" (Recommendation Edition 2015), covering its eight published scales (Z, N, TT, H0, S, 0, I, II); the 30%-above-minimum figure shown as a second result is NEM 302 §3. Both standards cover wagons/cars only, not locomotives. The scale ratios shown in parentheses are reused from this site's own Scale Converter, except Gauge II's (1:22.5), confirmed directly from MOROP NEM 010 "Scales, Nominal Sizes, Gauges" (Edition 2011) Table 1.
UK figures are not from either standard — no British national body publishes a wagon-weight standard (only uncitable forum estimates exist, e.g. ~150-200g per foot on RMweb). Instead, each UK scale is linearly interpolated between the two NEM 302 scales bracketing its actual ratio, reusing this site's own Scale Converter ratios, covering every UK scale within NEM 302's published range (1:22.5 to 1:160). Gauge 1 and G happen to match NEM's I and II ratios exactly, so those two are direct citations rather than estimates. The one UK scale outside that range — 16mm Scale (1:19.05) — is left out rather than extrapolated past real data.
Disclaimer: RP-20.1 and NEM 302 are recommended practices, not hard requirements — cars below target can still run fine on good track, and both standards note that track quality, wheel/coupler condition, and weight distribution also matter. The UK figures are estimates, not a standard; measure your own car's length and weight.