Window well cover sizes: measuring the well, not the window

By Francesco Zinghinì

A window well cover is sized to the well, not to the window inside it — and that distinction is where most wrong orders start. The well is the U- or rectangular-shaped liner (galvanized steel, plastic or masonry) that holds the soil back from a below-grade window; the cover has to span that liner and land on solid bearing, with enough overlap that runoff sheds outside the well instead of into it. Get three numbers right and almost any well takes a stock cover; measure the window instead, and nothing fits.

The three measurements

Measure the well itself, outside edge to outside edge:

• Width — along the foundation wall, from one side flange of the well to the other, at the widest point. Metal wells attach to the wall with flanges; include them, because the cover must clear or cap them.
• Projection — from the foundation wall out to the outermost edge of the well, again at the widest point. On a semicircular well this is the radius of the bulge; on a rectangular well, its depth away from the house.
• Height above grade — how far the well liner sticks up above the surrounding soil. A flat cover needs a reasonably even rim; a well that sits proud on one side and buried on the other wants a dome that can bridge the difference.

Take each measurement at the extreme point, the same smallest-versus-largest logic as the smallest-of-three method for windows, but inverted: a window must clear the tightest point of its opening, while a cover must span the widest point of its well.

Reading a cover size listing

Covers are listed by the well size they fit, not their own outside dimension — a cover for a 42 × 18 inch well is itself a few inches larger so it overlaps the rim. Stock metal and plastic wells cluster around widths of 36 to 72 inches and projections of 24 to 48 inches, and stock covers follow the same grid. If your measured width or projection lands between listed sizes, order the next size up: an oversized cover overlaps more rim and still sheds water, while an undersized one bridges nothing and falls in. Masonry and site-built wells are rarely on the grid at all; measure both dimensions at their widest and expect a custom or adjustable cover.

Cover types, briefly

• Clear domes (bubbles) — polycarbonate or acrylic, arched over the well. They shed rain and leaves, keep daylight coming through, and tolerate uneven grade. The default for most residential wells.
• Flat covers — polycarbonate sheets or sloped low-profile lids. Tidier looking, and the sloped versions still shed water; they want a level rim.
• Metal grates — steel grids that carry foot traffic and never yellow, but let rain and debris through. The choice where the well sits in a walkway and falling into it is the hazard being solved.

If people can walk over the well, the cover must be rated to carry them — look for a stated load rating rather than guessing from thickness.

The egress rule that overrides everything

If the window in the well is a bedroom or basement egress window, the cover is part of the escape route, and the IRC is blunt about it: a cover, grate or bar over an emergency escape opening must be releasable or removable from the inside without keys, tools or special knowledge, and the well below it must keep its required 9 square feet of clear area with 36 inches in each direction. A bolted-down grate or a snow-loaded dome that takes two hands and a crowbar fails that test and turns a code-compliant window into a trap. Deep wells carry one more requirement: past 44 inches of depth, the well needs a permanently fixed ladder or steps that the cover cannot block. Whether your window itself qualifies for egress is a separate check — run the numbers in the egress window calculator and the IRC R310 guide — but once it does, the cover must not undo it.

A worked example

Say the tape reads 44 inches along the wall (flange to flange), 20 inches of projection at the bulge, and the rim sits 2 inches proud of grade on the uphill side. The listing grid offers 42 × 18 and 48 × 24: the 42-inch cover is too small on both axes, so the order is the 48 × 24 — it overlaps the rim by a couple of inches all round and its dome bridges the uneven grade. If that well serves an egress bedroom window, the buying checklist adds one line: a cover that lifts or hinges from inside the well, no fasteners, no tools.

Common mistakes

• Measuring the window. The cover spans the well liner; the window size is irrelevant to the order.
• Measuring inside the flanges. The mounting flanges are the widest point of a metal well; a cover sized to the liner interior comes up short.
• Rounding down. Between stock sizes, the next size up overlaps; the next size down falls in.
• Bolting down an egress cover. Screwed-shut covers over escape openings fail inspection and, worse, fail in an emergency.
• Ignoring foot traffic. A dome in a side-yard path gets stepped on eventually; that location wants a load-rated grate or cover.

These are planning conventions, not a code sign-off: egress and well requirements are enforced by your local building department, and jurisdictions amend the IRC. Measure the well, order to the widest points, and where an escape opening is involved, let the release-from-inside rule pick the cover before looks do.

Reference standards for this topic are published by NFRC and ENERGY STAR; both are linked at section level, since their documents are revised over time.