Diameter to Radius Calculator
A diameter to radius calculator converts the full width of a circle into the distance from its center to its edge using the formula R = D ÷ 2. Enter the diameter to get the radius, circumference, and area instantly — whether you are sizing screws at the hardware store or planning a circular table from wood.
Diameter to Radius Conversion
The diameter to radius formula is R = D ÷ 2. The diameter is the longest straight line across a circle, passing through its center. The radius is exactly half that distance — the straight line from the center to any point on the edge. Divide any diameter by 2 to find the radius.
Precision Calipers
R = D ÷ 2Diameter to Radius Conversion Chart
| Radius | Diameter | Circumference | Area |
|---|---|---|---|
| 1 | 2 | 6.283 | 3.142 |
| 5 | 10 | 31.416 | 78.54 |
| 10 | 20 | 62.832 | 314.159 |
| 25 | 50 | 157.08 | 1963.495 |
| 50 | 100 | 314.159 | 7853.982 |
| 100 | 200 | 628.318 | 31415.927 |
| 250 | 500 | 1570.796 | 196349.541 |
| 500 | 1000 | 3141.593 | 785398.163 |
All values in standard units. Circumference = 2πR • Area = πR²
Diameter to Radius Ratio
The ratio between diameter and radius is always exactly 2:1. Every circle's diameter is twice its radius, and every circle's radius is half its diameter. This relationship holds for circles of any size — from a 3 mm (0.12 in) screw head to a 50-inch (127 cm) circular table: 3.14159265...
The 2:1 Bisection
The diameter is perfectly halved by the center point. One full diameter length mathematically splits into exactly two identical radius lengths.
Radius vs Diameter: What's the Difference?
Radius and diameter are 2 distinct measurements of a circle. The diameter is the full distance across a circle through its center — the longest straight line within the circle. The radius is exactly half: the distance from the center to the edge. Confusing diameter for radius (or vice versa) leads to errors, from picking wrong-sized screws at the hardware store to miscalculations in structural designs.
Hardware Store Sizing
Confusing diameter for radius when buying screws will result in fasteners that are exactly twice or half the required size. Select a screw gauge below to compare structural traits.
Diameter vs Radius of a Circle
Diameter and radius are directly proportional — when the diameter doubles, the radius doubles. The diameter is always exactly 2 times the radius, and the radius is always exactly half the diameter. Drag the slider to see diameter and radius scale together.
Ergonomic Table Sizing
A user's average arm reach is roughly 25 inches. If you buy a table based on its total diameter, you must halve it to ensure the radius does not exceed this comfortable reach distance.
How to Calculate the Radius from the Diameter
To calculate the radius from the diameter, divide the diameter by 2. The diameter to radius formula is R = D ÷ 2. A shop assistant tells you a circular tabletop has a fixed diameter of 50 inches (127 cm). The radius — the distance from the center to the edge — is 50 ÷ 2 = 25 inches (63.5 cm).
The Splitting Forge (÷2)
Activate the laser to physically cleave the diameter in half. This perfectly illustrates the transition from one $D$ segment into two independent $R$ segments.
Diameter to Radius: Worked Example
You are building a circular table and the shop assistant says the available wood tabletops have a fixed diameter of 50 inches (127 cm). You want everyone at the table to reach the center — so you need to know the radius.
CAD Draft View
Hover over the CAD model of the wooden table. The architectural software automatically divides the total 50-inch physical span by 2 to map the 25-inch ergonomic radius reach zones.