Guide
Scale model conversion: real size to model size
You have a real dimension — a bus, a boxcar, a shipping container — and want to know how long the model should be. Here is the formula, the mm-per-foot shortcuts that make it mental arithmetic, and a ready table for the popular railway scales. For any other ratio, the scale calculator does the division for you.
How the conversion works
A ratio like 1:76 means the model is 1/76th of the prototype in every dimension — so a real object 76 mm long becomes 1 mm of model. British modellers flip this into a friendlier unit: “4 mm scale” (OO) means every real foot becomes 4 mm, “3.5 mm scale” (HO) means every foot becomes 3.5 mm. Same arithmetic, more intuitive units — which is why UK ratios like 1:76 look odd on paper but feel natural on the workbench.
In millimetres the formula is one division: model size = real size ÷ denominator. A 9,449 mm double-decker bus at 1:87 comes out at 9,449 ÷ 87 = 108.6 mm. Keep the real measurement in mm first (multiply feet by 304.8), divide once, and round to a tenth of a millimetre — finer precision than any plastic kit deserves.
Measuring in feet? Skip the unit conversion entirely and multiply by the scale’s mm-per-foot figure: HO (1:87) ≈ 3.5 mm per foot, OO (1:76.2) = 4 mm per foot exactly, N (1:160) ≈ 1.9 mm per foot. The OO figure is exact by design — 4 mm to the foot is how the scale was defined — so a 60 ft coach is 60 × 4 = 240 mm with no calculator in sight. (The general figure is 304.8 ÷ denominator.)
Quick table: real size → HO / OO / N
Six everyday dimensions in the three most-modelled scales. HO at 1:87, OO at 4 mm to the foot (1:76.2), N at 1:160 — values rounded to 0.1 mm.
| Real-world item | Real length | HO (1:87) | OO (1:76.2) | N (1:160) |
|---|---|---|---|---|
| 40 ft shipping container | 40 ft (12,192 mm) | 140.1 mm | 160.0 mm | 76.2 mm |
| 50 ft freight wagon | 50 ft (15,240 mm) | 175.2 mm | 200.0 mm | 95.3 mm |
| 60 ft express passenger coach | 60 ft (18,288 mm) | 210.2 mm | 240.0 mm | 114.3 mm |
| Double-decker bus | ~31 ft (9,449 mm) | 108.6 mm | 124.0 mm | 59.1 mm |
| Commercial truck (rigid) | ~24 ft (7,315 mm) | 84.1 mm | 96.0 mm | 45.7 mm |
| Everyday car | ~15 ft (4,572 mm) | 52.6 mm | 60.0 mm | 28.6 mm |
Lengths are typical figures, not standards: buses and cars vary by model, and UK rolling stock often runs shorter than its US equivalent. For exact prototypes, measure once and scale it.
Worked conversions
- OO (4 mm scale), a 60 ft express coach. The mm-per-foot shortcut does it in one step: 60 × 4 = 240 mm. Prefer the formula? 60 ft = 18,288 mm, and 18,288 ÷ 76.2 lands on the same 240 mm.
- HO (1:87), a 40 ft shipping container. 40 ft × 3.5 ≈ 140 mm(exactly 12,192 ÷ 87 = 140.1 mm). Add a wagon underneath — a 50 ft car is 175.2 mm in HO — and you know instantly whether the train fits the shelf.
Planning a layout? Run every vehicle on the wish list through the same formula and the available wall length tells you how long a train you can actually hold.
Frequently asked questions
How big is a HO scale car?
An everyday road car is about 15 ft (4.6 m) long, and HO is 1:87 — so the model comes out around 52.6 mm, roughly 2 inches. The quick way to see it: HO is 3.5 mm to the foot, and 15 × 3.5 = 52.5 mm. Rail vehicles are longer: a 40 ft boxcar in HO is about 140 mm (5½ in) over couplers.
What size is a 40 ft container in HO or OO scale?
In HO (1:87), a 40 ft container is 40 × 304.8 ÷ 87 = 140.1 mm long — about 5½ in. In OO (4 mm to the foot) it is exactly 160 mm, because 40 ft × 4 mm = 160 mm — that foot-based shortcut is the whole reason OO was designed at 4 mm scale.
How do I convert a real-world size to a model size?
Work in one unit and divide by the ratio denominator: model size = real size ÷ denominator. A 12,192 mm (40 ft) container at 1:76 is 12,192 ÷ 76.2 = 160 mm. If you measure in feet, the shortcut is even faster — multiply feet by the scale's mm-per-foot figure (3.5 for HO, 4 for OO, about 1.9 for 1:160 N).
Why is British OO exactly 4 mm to the foot?
OO was defined around 4 mm representing one real foot, which fixes the ratio at 1:76.2 (4 mm × 76.2 = 304.8 mm = 1 ft). The mm-per-foot rule makes mental arithmetic trivial — every foot of prototype becomes exactly 4 mm of model — at the cost of an odd-looking ratio and a slightly narrow 16.5 mm track gauge.
Does the same formula work for N scale and other ratios?
Yes — the formula is universal: divide the real size by the denominator. Global N is 1:160 (about 1.9 mm per foot), British N is 1:148, TT is 1:120, O is 1:43.5 in the UK and 1:48 in the US. Only the denominator changes; the arithmetic is identical. For any custom or odd ratio, the scale calculator does the division for you.
Working in drawings and plans rather than railways? Open the free scale calculator →