How big should that terrain piece be? The scale math behind 28mm, HO, and 1:35 builds

Every terrain builder eventually hits the same wall: you find a great reference — a ruined farmhouse, a watchtower, a shipping container — with real-world dimensions listed in feet or meters, and you need to know how big to cut the foam. Eyeball it and the piece ends up subtly wrong on the table: a door your miniatures could never walk through, a "two-story" ruin that barely clears a model's shoulders, or a guard tower that looms like a skyscraper next to everything else you own. Scale conversion is a single division, but you have to divide by the right number — and the right number depends on which hobby you are building for.

Scale is a ratio, and every scale name hides one

Every miniature scale is really a ratio between real-world size and model size. A 1:60 scale means every real-world dimension gets divided by 60: a 30-foot building becomes a 6-inch model. The confusion comes from the naming conventions, because tabletop wargaming names its scales after the height of a human miniature instead of the ratio. "28mm scale" means a six-foot human stands about 28 millimeters tall, which works out to roughly 1:60. "32mm scale" — the slightly chunkier, more heroic proportion popular in modern skirmish games — is roughly 1:56. Meanwhile, model railroading and military modeling name scales by the ratio directly: HO scale is 1:87, and the classic military modeling scales are 1:35, 1:48, and 1:72. Same math, different vocabulary, and the difference between them is anything but cosmetic.

The same building is a very different size in each hobby

Take a 10-meter (roughly 33-foot) two-story building. At 28mm tabletop scale (1:60), it comes out to about 167 mm tall — a substantial centerpiece terrain piece around six and a half inches high. At HO scale (1:87), the identical building is about 115 mm — noticeably smaller, because railroad scale compresses harder. At 1:72, it is about 139 mm. Build that structure at railroad proportions for a wargaming table and it will read as cramped and toy-like next to 28mm miniatures; build it at 28mm proportions for a railroad layout and it dwarfs the trains. Mixing scales by even one step is visible instantly on the table, which is why the conversion is worth doing on paper before the knife touches the foam.

Doors, walls, and the details that sell the scale

Big dimensions are forgiving; small ones are not. A viewer cannot tell whether a hill is 3 percent too tall, but they can absolutely tell when a doorway is wrong, because there is a miniature standing right next to it for comparison. A standard 6-foot-8-inch real-world door is about 34 mm tall at 28mm scale and about 23 mm at HO scale. Window sills, wall thicknesses, fence heights, and vehicle clearances all follow the same rule: convert them individually rather than scaling one measurement and eyeballing the rest, because eyeballed proportions drift, and drifting proportions are exactly what makes a terrain piece look "off" in a way the builder can never quite diagnose.

From scaled millimeters to foam-board layers

Most scratch-built terrain is stacked from foam sheets — XPS insulation foam or foam board — that come in fixed thicknesses, commonly 12 mm or so. Once you know a piece's scaled height in millimeters, the number of layers is that height divided by the sheet thickness, rounded up. A 20-foot watchtower at 28mm scale converts to about 102 mm of height; with 12 mm foam, that is 102 ÷ 12 = 8.5, so nine layers, with the top layer sanded or carved down to hit the exact height. Rounding up rather than down matters for the same reason it does in any materials estimate: an extra millimeter of foam is a few seconds of sanding, while coming up one layer short means gluing a sliver on top and hiding the seam.

Worked example: a ruined farmhouse for the wargaming table

Say the reference farmhouse is 24 feet tall at the roof peak. Converting to millimeters first, 24 feet is about 7,315 mm of real-world height. At 28mm tabletop scale, divide by 60: the model should stand about 122 mm — just under five inches. With 12 mm foam sheets, that is eleven layers (122 ÷ 12 = 10.2, rounded up). Run the same reference at 1:56 for a 32mm skirmish game and the target becomes about 131 mm — around 7 percent larger, which sounds trivial but is precisely the difference that makes 32mm miniatures look correctly proportioned against it. The doorway, at a real-world 7 feet, should be about 36 mm tall at 1:60 — tall enough that a 28 mm miniature on its base passes under it believably.

Convert before you cut

The whole calculation is unit conversion, one ratio division, and one rounding step — trivial arithmetic that is nonetheless very easy to fumble when you are juggling feet, meters, inches, and millimeters across scales named in two different conventions. Our miniature scale and terrain converter takes a real-world dimension in feet, meters, inches, or millimeters, converts it to 28mm or 32mm tabletop, HO, 1:35, 1:48, or 1:72 scale, and tells you the scaled size in both millimeters and inches — plus exactly how many foam-board layers of your chosen thickness to stack. Run every key dimension through it before cutting, and the piece will sit next to your miniatures like it was always meant to be there.