Japanese Cake Size Numbers: How Many cm and Servings Are No. 5 and No. 6?
When you order a birthday cake in Japan or choose a pan for a homemade decorated cake, you'll always run into size numbers such as "No. 5" and "No. 6" (written 5-go and 6-go). The number indicates the diameter, with No. 1 equal to about 3 cm. This article gives a chart of sizes, centimeters and servings, tips for choosing a size by headcount, ingredient factors when changing size, and how to compare with square cakes.
Chart of size number, diameter and servings
A cake's size number indicates the diameter of a round cake. No. 1 is about 3 cm, so the number × 3 gives the diameter in centimeters. Servings are a guide for a strawberry shortcake cut to a typical size.
| Size | Diameter | Servings | Amount compared with No. 5 |
|---|---|---|---|
| No. 3 | 9 cm | 1–2 | ×0.36 |
| No. 4 | 12 cm | 2–4 | ×0.64 |
| No. 5 | 15 cm | 4–6 | ×1 |
| No. 6 | 18 cm | 6–8 | ×1.44 |
| No. 7 | 21 cm | 8–10 | ×1.96 |
| No. 8 | 24 cm | 10–12 | ×2.56 |
| No. 9 | 27 cm | 12–15 | ×3.24 |
| No. 10 | 30 cm | 15–20 | ×4 |
Each step up adds 3 cm to the diameter, but the amount of cake (area) grows more than you might expect: ×1.44 from No. 5 to No. 6, and ×1.36 from No. 6 to No. 7. Even "just one size up" makes a real difference once it's cut.
Why is one size 3 cm?
The size number comes from the sun, a unit in Japan's traditional shakkanho system of measures. One sun is about 3.03 cm, and cake pans were made in one-sun steps, so counting No. 1 as 1 sun, or about 3 cm, became established. Most pans today are labeled in centimeters and sold at rounded figures, such as No. 5 as 15 cm and No. 6 as 18 cm.
Even at the same size, height and the amount of decoration vary by shop. When ordering, check the diameter, the height and how many sponge layers there are, not just the number, to avoid a surprise. Another example of traditional units surviving in daily life is in Tatami Sizes and Room Measurements.
Tips for choosing a size by headcount
- For a family birthday with 3–4 people, No. 4 or No. 5. As an after-dinner dessert, No. 4 is enough.
- For 5–6 people, No. 5, or No. 6 if someone will want seconds.
- Gatherings with many children can use smaller slices, so count up to the top of the chart's range.
- For all adults with other food around, the bottom of the range is about right.
- For more than 8 people, two No. 5 cakes rather than one No. 7 give you two flavors and are easier to carry.
If who gets which piece becomes a lively question, you could also settle the order with ghost leg (amidakuji).
Ingredient factors for homemade cakes in another size
Homemade recipes are almost always written for No. 5 (15 cm) or No. 6 (18 cm). To make another size, multiply every ingredient by the square of the diameter ratio. In sponge eggs, that works out as follows (for a recipe using 2 eggs at No. 5).
| Size | Diameter | Factor from No. 5 | Factor from No. 6 | Eggs |
|---|---|---|---|---|
| No. 4 | 12 cm | ×0.64 | ×0.44 | About 1.3 |
| No. 5 | 15 cm | ×1 | ×0.69 | 2 |
| No. 6 | 18 cm | ×1.44 | ×1 | About 2.9 |
| No. 7 | 21 cm | ×1.96 | ×1.36 | About 3.9 |
| No. 8 | 24 cm | ×2.56 | ×1.78 | About 5.1 |
Decorations such as cream and fruit generally scale by the same factor, but cream spread on the sides grows with the circumference (the diameter ratio), so a little less than the factor is enough in practice. Concrete conversion examples and baking time adjustments are in Cake Pan Size Conversion.
What size is a square cake equal to?
Square cakes are easy to cut and suit big parties and potlucks. Round and square pans with the same base area bake the same amount of batter to the same thickness.
| Round | Base area | Square of equal area | Factor to bake as 18 cm square |
|---|---|---|---|
| 15 cm (No. 5) | About 177 cm² | About 13.3 cm square | ×1.83 |
| 18 cm (No. 6) | About 254 cm² | About 16 cm square | ×1.27 |
| 21 cm (No. 7) | About 346 cm² | About 18.6 cm square | ×0.94 |
| 24 cm (No. 8) | About 452 cm² | About 21.3 cm square | ×0.72 |
- A 15 cm square pan (225 cm²) is a little smaller than an 18 cm round pan, at ×0.88.
- An 18 cm square pan (324 cm²) holds almost the same amount as a 21 cm round pan.
- For the same number, a square pan is about ×1.27 larger than a round one (18 cm round to 18 cm square is ×1.27).
In square pans, heat reaches the corners quickly and the center lags, so lowering the temperature 10 °C from a round-pan recipe and baking a little longer gives an even result.
Same number, different amount: height and layers
The number only tells you the diameter, so with a different height or number of sponge layers, even two No. 5 cakes differ in how filling they are. A typical shortcake is about 7–8 cm high, with 2–3 sponge layers filled with cream and fruit. Tall cakes and two-tier cakes can be cut into more servings than the chart shows.
| Type of cake | Amount at No. 5 | How to count servings |
|---|---|---|
| Typical shortcake (7–8 cm high) | Baseline | As the chart shows |
| Tall cake (10 cm high or more) | About ×1.3 | Top of range + 1–2 people |
| Tart or cheesecake (3–5 cm high) | About ×0.5–0.6 | Around the bottom of the range |
| Two tiers (No. 5 base + No. 3 top) | About ×1.4 | About the same as No. 6 |
To make a tall cake at home, convert by volume including height rather than by base area. Enter a different pan height in the Cake Pan Size Converter and choose "Match batter amount" to get the factor for a deeper pan.
FAQ
- How many cm and servings is a No. 5 cake?
- No. 5 is 15 cm across and serves about 4–6. It's a size often chosen for a birthday cake for a family of 4.
- How many cm are No. 6 and No. 7?
- No. 6 is 18 cm across (6–8 servings) and No. 7 is 21 cm across (8–10 servings). No. 1 is about 3 cm, so the number × 3 gives the diameter.
- How much more is No. 6 than No. 5?
- No. 6 is about ×1.44 of No. 5. The diameter differs by only 3 cm, but area grows with the square of the diameter ratio, so it's over 40% more.
- How many servings is a No. 4 cake?
- No. 4 is 12 cm across and serves 2–4. As an after-dinner dessert, it suits 3–4 people; if the cake is the main event, about 2 is right.
- What size square pan matches an 18 cm round pan?
- About 16 cm square. An 18 cm round pan has a base area of about 254 cm², nearly the same as a 16 cm square (256 cm²).
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