---
title: "Model Railway Gradient & Helix Calculator | Diorama Fern"
description: "Rise and run to gradient in % and 1 in N and back, NEM 101 limits and transitions, NMRA RP-11 minimum curve radius, and helix rise per turn with clearance."
url: https://dioramafern.me/model-railway-gradient-calculator/
---

% · 1 in N · radius · helix

# Model railway gradient calculator

How steep is that climb, how tight can the curve be, and will the helix clear the track above?

## How to use it

Gradient is rise over horizontal run: 100 mm up over 5 m is 2%, or 1 in 50. Choose what to work out, fill in the other two, and switch the gradient box between percent and 1 in N. MOROP's NEM 101 recommends at most 2.5% (1 in 40) on standard-gauge main lines, 3.0% on branch lines and 0.4% in stations, with 3.3%, 4.0% and 0.5% for narrow gauge.

Where a gradient starts or ends, NEM 101 asks for the change to be rounded off over a length in multiples of the track gauge G: at least 6 G for a 2% change, 10 G recommended on branch lines and 14 G on main lines. The calculator scales that to your gradient and gauge.

The curve table is NMRA RP-11's minimum radius for each class of equipment, from old-time short wheelbases to long modern cars; use the largest radius your space allows. In a helix, one turn of track must climb at least the overhead clearance plus the thickness of the deck above, so the calculator compares the rise per turn with NMRA RP-7.1's bridge height for your era.

## Questions

**How do I turn 1 in N into a percentage?**

Divide 100 by N: 1 in 40 is 2.5% and 1 in 50 is 2%. To go back, divide 100 by the percentage.

**What radius does a helix need?**

Enough that one turn climbs past the clearance and deck at a gradient your trains can manage. At 2% in HO with classic-era clearance and a 15 mm deck, that is at least 732 mm to the track centre.

**Why is my train fine on the straight but stalls on the curve?**

A curve on a grade adds drag, and NEM 101 notes that tight radii on inclines cut the load a train can haul, so it advises not reaching the maximum gradient on tight curves. Ease the grade on curves or widen the radius.

## Sources

- Recommended maximum gradients and the rounding of gradient changes (300 G minimum vertical radius; 6, 7.5 and 12 G minimum for 2%, 2.5% and 4% changes): [MOROP NEM 101, Inclinations and changes of inclination](https://www.morop.eu/images/NEM_register/NEM_E/nem101_en_2025.pdf) (Recommendation, 2025).
- Minimum radius and turnout frog by equipment class for O, S, OO, HO, TT and N: [NMRA RP-11, Curvature and Rolling Stock](https://www.nmra.org/sites/default/files/standards/sandrp/General/RP/rp-11_2018.03.03r.pdf) (March 2018). RP-11 prints two cells where its inch and millimetre rows disagree (N class G: 10 in and 248 mm; OO class M: 33.75 in and 851 mm); both are shown as printed.
- Overhead clearance (dimension H, bridge height above the rail: 17, 22, 22 and 23 ft for the four eras): [NMRA RP-7.1, Tangent Track Centers and Clearance Diagrams](https://www.nmra.org/sites/default/files/standards/sandrp/General/RP/rp-7.1_tangent_track_centers_and_clearance_diagrams_2019.01.pdf) (January 2019).
- Track gauges: [MOROP NEM 010](https://www.morop.org/images/NEM_register/NEM_E/nem010_en_2011.pdf) (2011) and the targets in [NMRA S-3.2](https://www.nmra.org/sites/default/files/standards/sandrp/Mech/S/s-3.2_track_standard_scale.pdf) (2026).