Tatami to Square Meters (Regional)
Overview
A tatami mat is not one size. Four regional standards remain in use, and the conversion m² = mats × k requires knowing which applies. Kyoma, the Kyoto and Kansai standard, measures 191 × 95.5 cm for 1.824 m²; Chukyoma around Nagoya is 182 × 91 cm, or 1.656 m²; Edoma in Tokyo and the Kanto region is 176 × 88 cm, or 1.549 m²; and Danchima, developed for post-war public housing, is 170 × 85 cm, or 1.445 m². A six-mat room therefore spans 10.9 m² in Kyoto but only 8.7 m² in a Tokyo apartment block — a difference of more than twenty percent for an identical description. Japanese rental listings state the mat count as a shorthand for room size, so the regional standard, usually implied by the property's location, materially changes what a tenant is renting.
Variables
| Symbol | Name | Unit | Description |
|---|---|---|---|
| $tatami$ | Mat count | 畳 (jō) | Number of tatami mats describing the room. |
| $k_region$ | Regional mat area | m² | Area of one mat under the applicable regional standard. |
A Unit Without a Single Size
Japanese rooms are described by tatami mat count, but the mat itself has four regional standards still in use. The conversion therefore requires knowing which applies:
$$m^2 = tatami \times k_{region}$$
| Standard | Region | Dimensions | Area |
|---|---|---|---|
| Kyōma (京間) | Kyoto and Kansai | 191 × 95.5 cm | 1.824 m² |
| Chūkyōma (中京間) | Nagoya and Chūbu | 182 × 91 cm | 1.656 m² |
| Edoma (江戸間) | Tokyo and Kantō | 176 × 88 cm | 1.549 m² |
| Danchima (団地間) | Public and modern housing | 170 × 85 cm | 1.445 m² |
Every standard keeps a 2:1 length-to-width ratio, so two mats form a square and mats tile a rectangular room without cutting.
The Same Room, Different Sizes
A six-mat room spans 10.9 m² under the Kyoto standard but only 8.7 m² as danchima — a difference of more than twenty percent for an identical description. Since rental listings state mat count as a proxy for room size, the applicable standard materially changes what a tenant receives. Location usually implies the standard, but modern apartment buildings anywhere in Japan often use danchima.
Why the Standards Diverged
Kyoto measured rooms by the mat and then built walls around the resulting layout, a method called tatami-wari. Edo carpenters instead fixed the column grid first and cut mats to fit the remaining space, a method called kyōma-wari despite the name. The Edo mat is consequently smaller, and post-war public housing shrank it further to economise on floor area.
Derivation & History
The mat derives from the same ken module as the tsubo. A Kyoto mat is one half-ken wide and one ken long, that is $20/11 \times 10/11$ metre, giving 1.6529 m² in the pure module — but the Kyoto standard measures between column centres and adds the column width, producing the larger 1.824 m² figure in practice. The Edo method measures the clear span between columns and therefore subtracts column thickness, yielding 1.549 m². The two conventions bracket the ideal half-tsubo mat of 1.6529 m², which is almost exactly the Nagoya standard.
Worked Examples
Six-mat Tokyo apartment room
- Look up the mat area: Edoma = 1.5488 m²
- Multiply: 6 × 1.5488 = 9.29
Result: ≈ 9.3 m²
Same room count in Kyoto
- Kyōma = 1.82405 m²
- 6 × 1.82405 = 10.94
Result: ≈ 10.9 m², about 18% larger than the Tokyo equivalent
Edge Cases & Limitations
Half mats: Rooms are frequently 4.5 mats, using one half-size mat. The half counts as exactly half the standard area.
Western-style rooms: Listings describe carpeted or flooring rooms in mat equivalents even when no tatami is present, and those conversions almost always use the danchima or Edoma figure.
Unstated standard: When a listing gives both mat count and square metres, dividing the latter by the former recovers the standard actually used.
Ryūkyū and half-size mats: Borderless half mats laid in alternating directions are common in modern interiors and follow their own dimensions.
Real-World Applications
Rental and sale listings state room size in mats. Furniture retailers publish layout guides by mat count, air-conditioner capacity is specified by the mat count of the room it can serve, and traditional carpentry and tea-ceremony architecture treat the mat as the governing design module.