Circle & Diameter Gauge
Place round objects (rings, coins, pipes, lenses) on screen and adjust the circle to match the outer or inner edge.
Place your ring on the circle until the inner circle matches the inside rim of your ring.
| Inner Dia | US/CA | UK/AU | EU |
|---|---|---|---|
| 14.9 mm | 4 | H 1/2 | 47 |
| 15.7 mm | 5 | J 1/2 | 49 |
| 16.5 mm | 6 | L 1/2 | 52 |
| 17.3 mm | 7 | N 1/2 | 54 |
| 18.1 mm | 8 | P 1/2 | 57 |
| 19.0 mm | 9 | R 1/2 | 59 |
| 19.8 mm | 10 | T 1/2 | 62 |
| 20.6 mm | 11 | V 1/2 | 65 |
| 21.4 mm | 12 | X 1/2 | 67 |
Circular Geometry: Definitions, Formulas & Conversions
Measuring circular geometries accurately requires an understanding of how linear dimensions correlate with curved perimeters. Unlike straight objects where a standard edge ruler suffices, round objects often cannot be aligned flat against a single axis.
Distance from edge to edge passing through the exact center point.
Distance from the central origin point to any point along the circumference.
Total linear perimeter length if the circular boundary were unrolled flat.
Enclosed 2D surface area (or cross-sectional fluid flow area in pipes).
Bolt Circle Diameter (BCD) & Pitch Circle Diameter (PCD)
In mechanical engineering, automotive wheel fitment, bicycle chainrings, and industrial piping, mounting holes are uniformly distributed along a circular centerline known as the Bolt Circle Diameter (BCD) or Pitch Circle Diameter (PCD).
| Pattern Type | Formula (Opposite Holes, s) | Formula (Adjacent Holes, a) | Common Industry Applications |
|---|---|---|---|
| 4-Bolt Pattern | BCD = s | BCD = a × 1.4142 | Compact automotive wheels (4x100mm), mountain bike chainrings (104mm BCD). |
| 5-Bolt Pattern | — (No direct opposite hole) | BCD = a × 1.7013 | Passenger vehicle hubs (5x114.3mm, 5x120mm), road bike cranks (110mm / 130mm BCD). |
| 6-Bolt Pattern | BCD = s | BCD = a × 2.0000 | Truck/SUV wheel hubs (6x139.7mm), bicycle disc brake rotors (44mm ISO standard). |
Plumbing Standards: Nominal Pipe Size (NPS) vs. Actual Outer Diameter (OD)
One of the most frequent measurement surprises in DIY home improvement and piping installation is that plumbing pipe sizes do not correspond directly to their physical measurements. The historical Nominal Pipe Size (NPS) specification refers to nominal internal flow capacities. The outer diameter remains strictly standardized so fittings, valves, and threaded connections thread together securely:
| Nominal Size (NPS) | Actual Outer Diameter (Inches) | Actual Outer Diameter (Metric mm) | Common Household Use |
|---|---|---|---|
| 1/4 inch NPS | 0.540 in | 13.7 mm | Ice maker supply lines, pneumatic tubing |
| 3/8 inch NPS | 0.675 in | 17.1 mm | Sink faucet supply connections, flare lines |
| 1/2 inch NPS | 0.840 in | 21.3 mm | Standard residential water supply branches, gas lines |
| 3/4 inch NPS | 1.050 in | 26.7 mm | Main water distribution trunks, outdoor hose spigots |
| 1 inch NPS | 1.315 in | 33.4 mm | Water service entrances, commercial piping |
| 1-1/2 inch NPS | 1.900 in | 48.3 mm | Bathroom sink drain traps (P-traps), pool returns |
| 2 inch NPS | 2.375 in | 60.3 mm | Shower drains, main sewer vent stacks |
Everyday Round Objects for Rapid Visual Calibration
Need a rapid physical reference to confirm your circle ruler's accuracy? Compare your on-screen circle with these standard, legally mandated physical objects:
Frequently Asked Questions About Circle & Radial Measurements
How do I accurately measure my ring size using this online circle ruler?▼
Take an existing ring that comfortably fits the intended finger. Place the ring flat onto your screen over the blue circular gauge. Adjust the diameter slider or drag the outer handle until the glowing boundary line aligns exactly with the inside rim (inner edge) of your ring. The table on the right will instantly highlight your matching US, UK, and European ring sizes.
What is the difference between inner diameter and outer diameter?▼
Inner diameter (ID) measures the distance across the hollow inside opening of a ring, pipe, or washer. Outer diameter (OD) measures across the entire object including its wall thickness. When sizing jewelry rings or plumbing fittings, the inner diameter is standard. For caps, lens filters, or coin capsules, measure the outer diameter.
How does the concentric rings grid assist in measurement?▼
Clicking 'Concentric Rings' renders evenly spaced millimeter reference rings around the center bullseye. This allows you to quickly center round objects like coins, lenses, or bottle caps without having to manually line up off-center edges.
What is Bolt Circle Diameter (BCD or PCD) and how is it calculated?▼
Bolt Circle Diameter (BCD), also called Pitch Circle Diameter (PCD), is the diameter of the imaginary circular line passing through the center of all bolts or mounting holes on a bicycle chainring, car wheel hub, or industrial pipe flange. For a 4-bolt pattern, measure center-to-center distance (s) between opposite holes: BCD = s. For adjacent holes: BCD = s × 1.414.
Can I measure round objects larger than my computer or phone screen?▼
If an object is wider than your display screen, use our 'Online Ruler Camera — Image Measuring' tool (/image-measure). Take a top-down photograph of your large circular object with a credit card or coin placed next to it as a reference scale, and use the radial caliper tool to measure any size diameter in the photo.
Why does a 1/2-inch plumbing pipe have an outer diameter greater than 0.5 inches?▼
In North American and international plumbing, Nominal Pipe Size (NPS) originally referred to the approximate inside diameter of thick wrought-iron pipe. Today, a nominal '1/2-inch' pipe actually has a standardized outer diameter of 0.840 inches (21.3 mm) to ensure compatibility with standard threaded pipe fittings, regardless of pipe schedule wall thickness.