How to Measure a Spa Cover (Including Radius) for a Perfect Replacement Fit?

A spa cover is measured from the spa itself at its widest outside points, not from a waterlogged old cover — record overall length, width, corner radius, and skirt depth, then order slightly oversized.

For the full breakdown, see our best Spa Cover guide.

The easiest way to end up with a cover that never seals is to trust the one already sagging on the tub. Foam absorbs water, vinyl stretches, and a cover that has spent three winters outside is no longer the shape it was built to be. Ordering a replacement starts with the shell underneath it: a tape measure, a carpenter’s square, and about fifteen minutes.

Getting the numbers right matters more than most buyers expect. A cover cut to the exact shell dimension will sit tight to the rim instead of overlapping it, and a corner radius measured too large can leave the cover physically unable to reach the corner. Below is the order that works, from the widest points down to the small details that decide whether the lid actually closes.

What Surfaces Do You Measure On A Spa?

Measure across the widest exterior points of whatever structure supports the cover — the shell or cabinet on a freestanding spa, the outer edges of the ledge on a built-in or deck-level tub, and the widest outside point of the acrylic lip on a rounded shell.

The distinction matters because a deck-level spa hides its cabinet behind the surround. If you measure the visible opening rather than the supporting ledge, the cover arrives small. On a freestanding hot tub, run the tape past the cabinet edge, not along the water line.

How Do You Measure Length, Width, And The Corner Radius?

Length and width are straight runs across the spa at its outermost points, rounded up to the nearest half inch, with roughly half an inch to an inch of overlap added overall. The radius takes one extra tool: a carpenter’s square pressed against the straight sides of a rounded corner.

Here is the full sequence:

1. Length and width. Hold the tape level and measure across the widest outside points in both directions. Round each figure up to the nearest 1/2 inch, then plan for about 1/2 inch of overhang per side — roughly 1 inch larger than the spa overall.
2. Corner radius. Set a carpenter’s square against the outside of a rounded corner, aligning its two straight edges with the spa’s straight sides. Measure from the square’s inside corner to the point where the square first touches the curve. The two distances from that imaginary corner to the curve should match.
3. Round the radius down. A radius rounded down to the nearest whole inch generally still fits, because a smaller radius covers a larger corner. A radius that is too large is the error that ruins the fit — the cover can’t reach into the corner at all.
4. Paper method for odd corners. Trace the curve onto paper, complete the implied circle, measure its diameter, and divide by two. When uncertain, always choose the smaller radius.

With the three core figures in hand, matching them against real products is far easier — the tested spa cover roundup sorts models by shape and size so you can compare against your numbers directly.

Which Details Decide Whether The Cover Actually Fits?

Skirt depth, hinge placement, and clearance are the details that turn a correctly sized cover into one that closes properly. None of them are part of the length, width, and radius figures, and all of them get ordered separately or specified at checkout.

What Are The Common Measuring Mistakes?

The most expensive mistake is using a deformed old cover as the template, followed by ordering the exact shell size with no overlap. Both produce a cover that looks right on paper and fits badly in the yard.

• Trusting a waterlogged cover. If the old cover has sagged, absorbed water, shrunk, or warped, measure the spa directly and check the old cover only as a cross-reference.
• Skipping the overlap. A cover sized to the rim rather than slightly beyond it sits inside the lip instead of sealing over it.
• Ignoring clearance. An oversized cover can foul decking, rails, steps, or a cover lifter. Walk the perimeter before you order.
• Assuming rectangular rules apply. Square, round, octagonal, and irregular spas don’t follow length/width/radius instructions for rectangular covers.
• Forgetting seller rounding rules. Rounding conventions vary by manufacturer — one guide rounds dimensions up to the nearest 1/2 inch, and covers 1–2 inches oversized often fit well.

The Gulf Stream Spas measuring guide and the Hot Spring Spas cover measurement page both walk through the corner-radius method with diagrams, and the Costco spa cover sizing document lays out the rounding and oversize thresholds in writing.