円計算ツール
The detailed guide below is currently available in English.
One measurement fixes the whole circle
A circle is fully determined by any single one of its four headline measurements, because each is a fixed multiple of the radius. The diameter is twice the radius; the circumference is 2π times it; the area is π times its square. That is why this calculator only asks which value you have — it converts that value to a radius internally, then rebuilds the other three from it.
Where each measure shows up in practice
Rolling distance and material wrapped around a cylinder follow the circumference; paint, turf, or pizza dough follow the area; bolts, lids, and pipe sizes are usually quoted as a diameter. Since area grows with the square of radius, a 35 cm pizza holds more than double the dough of a 25 cm one — a quick check this calculator makes easy.
よくある質問
Why does entering an area give different-looking digits?
Area entries are converted with a square root — r = √(A/π) — so small rounding in the area can shift several digits of the radius. Entering extra decimals in the area keeps the recovered radius close to the original.
Which value of π does the calculator use?
The full double-precision value of π built into JavaScript, which is far more accurate than any rounded value. Results are displayed to a few decimals, but the internal math keeps full precision.