How to Measure the Radius of a Hot Tub or Pool Cover

When you’re ordering a replacement hot tub cover or safety pool cover, getting the measurements right is key to ensuring a proper fit. One of the most common sticking points is the corner radius—the rounded corners on your cover. If the radius is measured incorrectly, the new cover may not sit properly, leaving gaps that reduce efficiency and protection.

Don’t worry—measuring the radius is simpler than it sounds. Here’s how you can do it using a carpenter’s square and a few other easy methods.

Why the Corner Radius Matters

Most hot tub and pool covers aren’t made with sharp square corners—they have rounded edges. The radius is the distance from the “square corner” to where the curve begins. Measuring it correctly ensures that your new cover lines up perfectly with the shape of your spa or pool.

Method 1: Using a Carpenter’s Square (Most Accurate)

A carpenter’s square, also called a framing square, makes this process quick and precise.

Step-by-step:

1. Place the carpenter’s square on the cover corner so the inside edges are flush against the straight sides of the cover.
2. You’ll see the rounded corner fall inside the square, leaving a gap.
3. Use a tape measure to measure from the inside corner of the square (the 90° angle) to the point where the cover’s curve touches the square.
4. That distance is your radius. For example, if the distance measures 4 inches, you have a 4-inch radius corner.

Method 2: Straightedge Measurement

Another way to measure is by using two straightedges (like rulers or boards).

Steps:

1. Place one straightedge along one side of the cover and another along the adjacent side so they meet at a 90° corner.
2. Measure from the intersection point of the straightedges (the imaginary “square corner”) to the start of the curve on either side.
3. That measurement equals your radius.

Tips for Accurate Measuring

• Measure at least two different corners—sometimes covers aren’t perfectly symmetrical.
• If your measurement falls between two standard radii, round up to the next size.
• The most common hot tub cover radii are 2", 4", 6", 8", and 10".
• Many suppliers (including us) offer printable radius charts that you can hold up to your cover for an easy comparison.

Get the Perfect Fit With National Discount Pool Supplies

Now that you know how to measure your cover’s radius, you’re ready to order a perfect-fitting replacement. At National Discount Pool Supplies, we specialize in high-quality spa and pool covers, including custom safety covers built to match your exact dimensions.

With the right fit, you’ll protect your pool or spa, extend your cover’s life, and enjoy peace of mind all season long.