Calculator
HO scale calculator
Convert a real-world size to HO — or a model measurement back to full size. HO is 1:87 in the European NEM standard (the NMRA in America uses 1:87.1); pick another scale or type any ratio.
Your answer appears here
Example: a 20 m long coach is about 230 mm long in HO at 1:87.
The same real size in other scales
| Scale | Ratio | Model size |
|---|---|---|
| HO (European NEM standard) | 1:87 | — |
| HO (American NMRA standard) | 1:87.1 | — |
| OO (British 4 mm to the foot) | 1:76.2 | — |
| TT | 1:120 | — |
| N | 1:160 | — |
| Z | 1:220 | — |
| S | 1:64 | — |
| O (European NEM standard) | 1:45 | — |
| O (7 mm to the foot) | 1:43.5 | — |
| O (American NMRA standard) | 1:48 | — |
| Gauge 1 (NEM) | 1:32 | — |
Enter a measurement above to fill in the table.
Notes
- HO (European NEM standard): NEM 010 gives HO (H0) as 1:87 on 16.5 mm track.
- HO (American NMRA standard): The NMRA lists HO as 1:87.1.
- OO (British 4 mm to the foot): The NMRA lists OO as 1:76.2; NEM 010 describes 4 mm scale as 1:76.
- TT: NEM 010 and the NMRA both give TT as 1:120.
- N: NEM 010 and the NMRA both give N as 1:160.
- Z: NEM 010 and the NMRA both give Z as 1:220.
- S: NEM 010 and the NMRA both give S as 1:64.
- O (European NEM standard): NEM 010 gives O (0) as 1:45, noting 1:43.5 is also used in some countries.
- O (7 mm to the foot): NEM 010 describes 7 mm scale as 1:43.5.
- O (American NMRA standard): The NMRA lists O as 1:48.
- Gauge 1 (NEM): NEM 010 gives nominal size I as 1:32.
Want the background? Read our guides on getting started, including why British OO differs from HO.
Where the numbers come from
Scale ratios are taken from these standards. Checked 18 October 2026.