AI-generated illustration, not a photograph of a specific product
"What radius will fit on my table?" is the question everyone asks before laying their first oval. It is also the wrong question to ask first. The right one is "what radius will my trains actually go round?" Tight curves save space, which is exactly why people buy them — and then discover their Shinkansen binding up on the first bend.
This page gives you the full KATO and TOMIX curve range with the baseboard size each one implies, in millimeters and inches, plus what fits on the two boards most people actually build on.
Once you have a radius in mind, you can lay the actual loop out in the free track planner — it snaps real KATO and TOMIX geometry, takes board dimensions in inches or millimeters, and tells you if the plan closes. Still deciding between the two systems? See KATO vs TOMIX.
Reading the codes: R315 and C280 are both millimeters
KATO prefixes its radii with R (R315 = 315 mm radius); TOMIX uses C (C280 = 280 mm radius). Despite the different letters, both are the radius of the track centerline in millimeters — there is no hidden difference in how they are measured.
A full circle therefore measures 2 × radius across the centerline. Your board has to be wider than that, because the sleepers, the roadbed edge and a margin against derailments all sit outside the centerline:
Board width = (2 × radius) + (2 × edge clearance)
Board length = (2 × radius) + straight section + (2 × edge clearance)
In an oval the two straights run parallel, so the straight length is added once, not twice.
Keep one thing in mind: this only answers "does the track geometry fit". It does not include roadbed overhang, the body overhang of long cars swinging out on a curve, catenary masts, platforms, or the space your hands need to reach in. Budget extra for all of those.
KATO Unitrack: R150 to R381
The board figures below assume 30 mm of clearance per side (so 60 mm on top of the track diameter). That 30 mm is an illustrative figure you can increase — it is not a KATO-published minimum clearance.
| Radius | How KATO positions it | Diameter (2R) | Board width (30 mm/side) |
|---|---|---|---|
| R150 | Compact; very tight, restricted stock | 300 mm | 360 mm (14.2 in) |
| R183 | Compact; very tight, restricted stock | 366 mm | 426 mm (16.8 in) |
| R216 | Standard curve section | 432 mm | 492 mm (19.4 in) |
| R249 | The default minimum where nothing is stated | 498 mm | 558 mm (22.0 in) |
| R282 | Clears most conventional-line stock | 564 mm | 624 mm (24.6 in) |
| R315 | KATO's baseline radius; suits most models | 630 mm | 690 mm (27.2 in) |
| R348 | Standard curve section | 696 mm | 756 mm (29.8 in) |
| R381 | Recommended by KATO for realistic scenes and high-speed running | 762 mm | 822 mm (32.4 in) |
TOMIX Fine Track: C103 to C391
TOMIX groups C103 as Super Mini Curve and C140/C177 as Mini Curve. None of the three is universal — TOMIX publishes a separate list of vehicles cleared to run on them, and you should check it before buying.
| Radius | How TOMIX positions it | Diameter (2R) | Board width (30 mm/side) |
|---|---|---|---|
| C103 | Super Mini; check the cleared-vehicle list | 206 mm | 266 mm (10.5 in) |
| C140 | Mini; check the cleared-vehicle list | 280 mm | 340 mm (13.4 in) |
| C177 | Mini; check the cleared-vehicle list | 354 mm | 414 mm (16.3 in) |
| C243 | Standard curve section | 486 mm | 546 mm (21.5 in) |
| C280 | Standard curve section | 560 mm | 620 mm (24.4 in) |
| C317 | Standard curve section | 634 mm | 694 mm (27.3 in) |
| C354 | Standard curve section | 708 mm | 768 mm (30.2 in) |
| C391 | Standard curve section | 782 mm | 842 mm (33.1 in) |
What fits a 4x8 sheet, and what fits a door
Most people do not build on an arbitrary rectangle — they build on whatever sheet material is easy to get. Two boards dominate, so here is where each one lands:
| Board | Short dimension | Largest radius that fits (30 mm/side) |
|---|---|---|
| 4 × 8 ft plywood sheet | 48 in / 1,219 mm | Everything — up to roughly 579 mm radius, so R381 and C391 both fit with room to spare |
| 30 × 80 in hollow-core door | 30 in / 762 mm | KATO R348 (756 mm board) just fits; R381 does not. TOMIX C317 (694 mm) is comfortable; C354 at 768 mm is already over |
| 2 × 4 ft board | 24 in / 610 mm | KATO R249 (558 mm) and TOMIX C243 (546 mm); R282 at 624 mm does not fit |
Two things worth drawing out of that table. First, on a 4x8 the curve is never the limiting factor — you can run the largest radius either maker sells and still have width left over, so the real design question is how much scenery, how many tracks and what kind of station you fit alongside it. Second, the hollow-core door is the tightest common board: it lands right on the boundary where the big radii stop fitting, which is exactly why door layouts so often end up at R348/C317.
Measure your own material before committing. Doors in particular vary — 28, 30, 32 and 36 in are all sold, and the 2 in difference decides whether R348 fits.
Our take: which radius to actually choose
The chart tells you what fits. Here is what we would pick:
- Short conventional-line stock (commuter EMUs, two or three cars): R249 / C243 is the sweet spot. It needs a board around 22 in deep and everything looks right on it.
- Shinkansen or full-length coaches: R315 / C280 as a floor, R348–R381 if you have the room. Long bodies overhang badly on tight curves and the angle between cars becomes obvious; a bigger radius is not just prettier, it runs better at speed.
- Genuinely tight on space (Compact / Mini territory): workable, but only with the short stock on the maker's cleared list. Do not expect long trains to cope.
The point people most often miss: a stated minimum radius is a floor, not a recommendation. "Clears R249" means it will just get round — not that R249 is where it looks its best. Whenever the board allows it, go a size up. A few extra centimeters buy smoother running and a much better look for very little money.
Calculator: work out your footprint
Enter a radius, the length of each straight and the clearance you want per side. The result is the outer rectangle of a single-track oval, given in millimeters and inches. It does not include double-track spacing, turnouts, platforms or scenery.
Do one physical check before you commit: tape the outline onto the actual table, lay the outermost curve, and run your longest vehicle round it slowly. Five minutes of masking tape has saved a lot of people from ordering the wrong track.
Five mistakes worth avoiding
- Treating the minimum as the target. "It clears" is the floor, not the best-looking option.
- Assuming N scale is one specification. Body length, bogie spacing and couplers all differ — check the individual model. Steam locomotives vary most of all, because their fixed driving-wheelbase cannot swivel.
- Cutting the board at 2R. That is the centerline diameter only, with nothing left for roadbed or overhang.
- Forgetting the straights. An oval is not a circle; once you add a platform or a passing loop it is usually the length that runs out first.
- Designing single track and adding the second later. Track spacing and turnouts change the outer dimensions — plan both from the start.
Sources
Checked 2026-08-01. Radii, product positioning and small-radius vehicle restrictions follow KATO and TOMIX official material; board widths and lengths are geometric calculations from the formula above. Inch figures are converted at 25.4 mm and rounded to one decimal. Re-check the official pages and the model's instruction sheet before ordering.
- KATO Unitrack curved track (R216–R381 and Compact)
- KATO Unitrack Compact (R150, R183 and their vehicle restrictions)
- TOMIX Fine Track range / Super Mini and Mini Curve cleared-vehicle list
Turn a radius into a plan that actually fits
Frequently asked questions
What minimum radius should I use for N scale?
Work in this order: check the train first (the minimum radius printed on its product page, box or instruction sheet), then pick the curve, then size the board. Your layout has to satisfy whichever locomotive or car demands the largest radius. A board size calculation only tells you whether the geometry fits — it never tells you whether a particular train will go round.
Will every N scale train run on KATO R150 and R183?
No. KATO sells R150 and R183 as Unitrack Compact and states they suit only certain small vehicles — longer stock is not guaranteed to make it round. Check each model's instruction sheet; where nothing is stated, KATO's guidance is to treat R249 as the minimum. Do not put long trains such as Shinkansen or full-length coaches on these curves.
Will every train run on TOMIX C103, C140 and C177?
No. TOMIX classes C103 as Super Mini Curve and C140 and C177 as Mini Curve, and publishes a dedicated list of vehicles cleared to run on them. If a model is not on that list and its packaging and instructions say nothing, do not assume it will pass — even if the geometry fits your board.
How do I calculate the baseboard size for an N scale oval?
Board width = (2 x radius) + (2 x edge clearance). Board length = (2 x radius) + straight section + (2 x edge clearance). That is pure geometry: it does not account for roadbed width, the overhang of long cars on curves, catenary masts, platforms, or room for your hands. Add margin on top of the figure.
What radius fits on a 4x8 sheet of plywood?
The 4 ft dimension is the constraint: 48 in is 1,219 mm, so with 30 mm of clearance each side you have room for a radius up to roughly 579 mm. Every curve either maker sells fits comfortably — even KATO R381 (822 mm board) and TOMIX C391 (842 mm) leave plenty of margin. On a 4x8 the interesting question is not whether a curve fits but how much scenery and how many tracks you can get alongside it.
What radius fits on a hollow-core door?
A common 30 x 80 in door is 762 x 2,032 mm. With 30 mm clearance each side, KATO R348 needs a 756 mm board and just fits; R381 needs 822 mm and does not. On the TOMIX side C354 needs 768 mm, which is already marginally over — C317 at 694 mm is the comfortable choice. Measure your actual door; sizes vary.
Why might a train still derail when the board size says it fits?
Because the formula answers a geometry question, not a clearance question. Whether a given model gets round depends on its length, bogie spacing, coupler type and how far the body overhangs on a curve. Always check the manufacturer's stated minimum radius or compatibility list for the specific model before buying track.
Is the stated minimum radius the radius I should build?
No — it is the floor, not a recommendation. When a maker says a model clears R249, that means it will just get round, not that R249 is where it looks or runs its best. If you have the space, go one or two steps larger. The few extra centimeters buy noticeably smoother running and far less body overhang on long stock.