How to Measure Hinge Radius Before You Order
A replacement hinge can have the right finish, leaf size, and screw pattern yet still fail to sit flush if its corner shape does not match the door’s existing mortise. Knowing how to measure hinge radius before you order prevents that frustrating mismatch and helps protect a clean, professional-looking installation.
Hinge radius refers to the curve at the corners of a hinge leaf. A square-corner hinge has sharp 90-degree corners. A radius-corner hinge has rounded corners designed to fit a routed mortise in the edge of a door or frame. The two most common residential rounded-corner sizes are 1/4-inch radius and 5/8-inch radius, but you should always measure rather than assume.
What Hinge Radius Means
The radius is the distance from the center of an imaginary circle to the rounded corner of the hinge. It is not the overall width of the hinge corner and it is not the same as the hinge leaf thickness.
This detail matters because door mortises are cut to a specific shape. If you install a 5/8-inch radius hinge in a 1/4-inch radius mortise, the rounded corner will be too broad for the cutout. If you install a 1/4-inch radius hinge in a larger 5/8-inch mortise, it may leave visible gaps at the corners. A square-corner hinge will not fit a rounded mortise without modifying the wood.
On a new door, you can choose the corner style that suits the installation method. On an existing door, the mortise usually determines the hinge radius you need.
How to Measure Hinge Radius Accurately
The most reliable approach is to remove one existing hinge and inspect the leaf that sits in the door edge. Measuring the hinge itself is generally easier and more accurate than measuring inside a painted or worn mortise.
Open the door enough to access the hinge screws, support the door if needed, and remove one hinge at a time. For a heavy exterior, solid-core, or commercial door, do not remove all hinges at once. The door weight needs to remain safely supported.
Start by Identifying the Corner Type
Lay the hinge on a flat surface and look at each corner. If the edges meet at a crisp angle, you need a square-corner hinge. If the corner follows a smooth curve, it is a radius-corner hinge.
Do not let paint buildup confuse the result. Layers of paint can make a rounded corner appear nearly square, particularly on older doors. Look at the unpainted edge of the hinge leaf or gently clean away loose paint from the corner before measuring.
Use a Radius Gauge When Available
A hinge radius gauge is the quickest option. Place the gauge against the rounded corner and find the cutout that follows the curve with no daylight or rocking. A proper match touches the curve evenly from one end of the arc to the other.
Many installers keep a small set of radius gauges because the tool also helps when matching cabinet hinges, metalwork, and routed trim. If you are replacing several hinges throughout a project, it is a worthwhile way to reduce guesswork.
Compare the Corner to Common Sizes
Without a radius gauge, measure the distance across the rounded corner from one point where the curve begins to the other. This is called the chord. Then measure from the midpoint of that straight line to the deepest point of the curve. This shallow measurement is the rise, also called the sagitta.
For a precise result, use this formula:
`Radius = (chord squared ÷ 8 × rise) + (rise ÷ 2)`
Because hinge corners are small, this method works best with a fine rule, calipers, or a digital measuring tool. It is more precise than trying to estimate the radius by eye, although it can be unnecessary when the hinge clearly matches a standard 1/4-inch or 5/8-inch profile.
A practical alternative is to trace the hinge corner onto paper, then compare that tracing to a known 1/4-inch and 5/8-inch radius template. The curve should align without a noticeable gap. When the existing hardware is old or irregular, a tracing can be more useful than a measurement taken over damaged finish.
Measure the Door Mortise Too
Before ordering, check the matching cutout in the door edge and frame. The hinge leaf should sit flat in both locations, with its edge flush to the surface. If the door and frame have different corner styles, one of them may have been modified during an earlier repair.
This is especially common in remodels, where a previous installer may have squared a rounded mortise with a chisel or filled gaps with wood putty. In that situation, select the hinge style that fits the door and frame with the least alteration, or plan for careful mortise repair rather than forcing a new hinge to fit.
Do Not Stop at the Radius
Hinge radius is only one part of getting the right replacement. A hinge with matching corners can still be incompatible if its dimensions or configuration differ from the original. Measure the open hinge from leaf edge to leaf edge to determine overall size. Common residential sizes include 3-1/2 by 3-1/2 inches and 4 by 4 inches, but larger doors often require larger hinges.
Also compare leaf thickness, screw-hole layout, number of knuckles, and whether the hinge uses plain or ball-bearing construction. Ball-bearing hinges are often a better choice for heavier or frequently used doors because they help reduce wear and improve movement. They may cost more than standard residential hinges, but the upgrade can make sense for solid-core, oversized, or high-traffic doors.
Finish should be the final matching step, not the first. Satin nickel, polished chrome, matte black, oil-rubbed bronze, and brass-toned finishes can vary between manufacturers. If you are replacing only one hinge, a close finish match may be difficult even when the listed color name sounds identical.
Common Measuring Mistakes
The most common error is measuring the outside width of the curved corner and calling that number the radius. The curve’s width is not its radius. Another frequent mistake is choosing a hinge based on a product photo. Photos rarely show enough detail to distinguish 1/4-inch radius from 5/8-inch radius accurately.
Buyers also sometimes measure only the door-side mortise. The frame-side mortise matters just as much. A hinge needs to seat properly on both sides to maintain consistent door alignment and reveal spacing.
Avoid assuming that all interior doors use 1/4-inch radius hinges or that all newer doors use 5/8-inch radius hinges. Both styles remain common, and square-corner hinges are still widely used. The correct answer depends on the door preparation in front of you.
When a Different Radius Can Work
Changing hinge radius is possible, but it is a modification project rather than a direct replacement. Converting a rounded mortise to square corners requires carefully chiseling the wood and may expose unfinished areas around the hinge. Converting square corners to a radius requires routing or reshaping the mortise with a consistent curve.
For a painted interior door, minor changes can sometimes be touched up successfully. For a stained, veneered, fire-rated, or commercial door, altering the mortise may be more noticeable or may affect the door’s rating and warranty. In those cases, matching the existing hinge preparation is usually the better choice.
A well-fitted hinge should disappear into the door edge: flat, aligned, and free of gaps that catch the eye. Take a few extra minutes to measure the radius, confirm the leaf size and hole pattern, and compare the door and frame mortises before placing the order. That small check is often what turns a replacement from “close enough” into the right fit.