Complete Guide to the Wood Volume Unit “Sai (才)”
From On-Site Shortcut Formula to Traditional Korean Calculation Methods
If you work with lumber, you’ve probably encountered the unit “sai (才)”—a traditional volume measurement that still determines wood pricing in many regions today.
This guide breaks down everything you need to know about sai: how it’s defined, how to calculate it quickly using the “300 conversion rule”, and how traditional Korean units such as 재 (jae) and 결 (gyeol) relate to it.
1. What Is “Sai (才)”?
Sai (才) is a traditional unit used to measure the volume of wood.
It originated during the Japanese colonial era and is still commonly used in lumber markets across Korea and parts of East Asia.
| Classification | Description |
|---|---|
| Definition | Volume of a wooden piece |
| Reference Size | Width 1 chi × Height 1 chi × Length 12 ja |
| Metric Equivalent | 1 sai ≈ 0.0033375 m³ |
| 1 chi (寸) | ≈ 3.03 cm |
| 1 ja (尺) | ≈ 30.3 cm (12 ja ≈ 3.636 m) |
2. The “300 Shortcut Formula” (for Metric Dimensions)
When you have lumber dimensions given in meters, you can easily convert to sai using this field-friendly rule:
Sai = Width × Height × Length × 300
This “×300 rule” is widely used on construction sites for quick volume estimation without converting units manually.
3. Traditional Calculation Method (Using Chi and Ja)
If you’re using traditional Korean or Japanese dimensions such as chi (寸) and ja (尺), use this formula:
Sai = (Width × Height × Length in ja) ÷ 12
For round logs, the rule changes slightly:
Sai = (Diameter² × Length in ja) ÷ 12
4. Traditional Korean Wood Volume Units
| Unit | Usage | Description |
|---|---|---|
| 재 (Jae) | Volume unit | Korean traditional unit for measuring wood volume (similar to sai, but locally defined) |
| 결 (Gyeol) | Bundle unit | 100 jae grouped together for bulk transactions |
⚠️ Note:
“Sai (才)” is of Japanese origin, while Jae (재) and Gyeol (결) are indigenous Korean units.
5. Important Tips When Calculating Sai
- Keep Units Consistent:
Don’t mix metric and traditional units. Stick to either meters or chi/ja. - Round Up Lengths:
When trading in ja units, decimals are usually rounded up. - Account for Saw Loss:
Sawing can remove 3–5 mm of material (“bubikki” loss), so include a small margin.
6. Example: Buying Lumber at a Sawmill
Scenario:
You plan to buy 10 lumber pieces.
- Dimensions: Width 10 cm × Height 5 cm × Length 3 m
- Volume (m³): 0.1 × 0.05 × 3 = 0.015 m³ per piece
- Sai Conversion: 0.015 × 300 = 4.5 sai per piece
Quick Reference Table
| Item | Description |
|---|---|
| Sai Standard | 1 chi × 1 chi × 12 ja = 1 sai |
| 1 Sai in m³ | ≈ 0.0033375 m³ |
| Metric Formula | Width × Height × Length × 300 |
| Traditional Formula | (chi × chi × ja) ÷ 12 |
| Round Log Formula | (diameter² × ja) ÷ 12 |
| Units to Remember | 재 (Jae), 결 (Gyeol) |
| Cautions | Consistent units, rounding, saw loss |
Final Thoughts
With this guide, sai calculations no longer need to be confusing.
Whether you’re estimating materials at a construction site or preparing a quotation, the 300 formula and traditional methods will help you calculate quickly and accurately.
This information is intended to assist anyone involved in timber trade, carpentry, or construction, especially when navigating between metric and traditional systems.
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