Measuring a hinge radius is key to replacing or installing doors and hardware correctly. The radius refers to the curve in the corner of the hinge plate where it fits into the routed or mortised area on the door and frame.
Quick Method: The Quarter Trick
The simplest and most common way to estimate the hinge radius, especially for the popular sizes, involves using a standard U.S. quarter.
According to common practice, the best way to determine what radius you have is by the following technique. Place a quarter in the corner of the door hinge. If the radius of the quarter is roughly the same as the quarter, you have a 5/8" round.
- How to do it:
- Open the door slightly to expose the hinge plate.
- Take a U.S. quarter coin.
- Carefully place the edge of the quarter into the curved corner of the hinge plate.
- Observe how well the curve of the quarter matches the curve of the hinge corner.
- If the curves align closely, it indicates a 5/8-inch radius hinge.
This method is quick and effective for identifying one of the most common hinge sizes.
Other Common Radii
While the 5/8" radius is prevalent, especially in residential construction, another common size is the 1/4-inch radius.
- 1/4-inch Radius: This curve is much tighter than a quarter's radius. If a quarter seems too large and doesn't fit snugly into the corner curve, you likely have a 1/4-inch radius. A dime or penny is still too large for this radius.
Some less common radii exist, such as square corners (no radius) or even larger 1/2-inch radii, though these are less frequent.
Precise Measurement Methods
For greater accuracy, especially when the quarter trick isn't definitive or the radius seems unusual, you can use specialized tools:
- Radius Gauges/Templates: These are sets of curved metal or plastic templates labeled with specific radius measurements (e.g., 1/4", 5/8", 1/2"). You simply find the template that fits the hinge corner curve perfectly.
- Profile Gauge (Contour Gauge): While not specifically for hinges, a profile gauge can capture the exact shape of the corner, which you can then compare to known radius dimensions or measure directly on the captured profile.
- Digital Calipers: With careful placement, digital calipers can sometimes be used to estimate the chord length of the curve, from which the radius can be calculated, though this requires some geometric understanding and is often overkill.
Understanding Hinge Radius in Context
The hinge radius is crucial because the door and frame are typically routed out (mortised) to exactly match the shape of the hinge plate, including the corners. Using a hinge with the wrong radius will prevent it from sitting flush, affecting the door's fit, function, and appearance.
| Hinge Corner Type | How to Identify | Common Use |
|---|---|---|
| 5/8" Radius | Quarter fits snugly in the corner | Very Common (Residential) |
| 1/4" Radius | Quarter is too large, tighter curve | Common (Residential & Commercial) |
| Square Corner | No curve, sharp 90-degree angle | Less Common (Older doors, some commercial) |
| Other Radii | Require specific measurement (gauge) | Less Common |
By using the quarter trick or a radius gauge, you can quickly and accurately determine the hinge radius needed for your door project.
