Window Well Gravel Calculator

About the Window Well Gravel

This calculator estimates how much drainage gravel is needed to fill a basement window well, based on the well's width, its front-to-back depth, and how high the gravel should be filled. It is meant for homeowners or contractors installing or refreshing a window well who need to know roughly how many cubic yards or tons of gravel to order before a delivery.

How It Works

You enter the window well's width (the dimension running along the foundation wall), its depth (how far the well projects out from the wall, front to back), and the fill height you want the gravel to reach, typically a few inches below the window sill. The calculator treats the well's footprint as a curved, semicircle-like bow rather than a straight-sided box, since most prefabricated window wells are rounded rather than square. It then multiplies that footprint area by the fill height to get a volume, converts that volume to cubic yards, and estimates the equivalent weight in tons using a drainage-gravel density figure.

Volume (cu ft) = (pi x Width(ft) x Depth(ft) x FillHeight(ft)) / 4; Cubic Yards = Volume(cu ft) / 27; Weight (tons) = Cubic Yards x 1.4

Formula & Methodology

Internally the tool first computes the area of a semicircle whose radius equals the well's depth: (pi x Depth^2) / 2. It then scales that area by the ratio of the actual width to twice the depth (Width / (2 x Depth)), which adjusts the shape for wells that are wider or narrower than a perfect semicircle. Multiplying by the fill height converts that adjusted area into a volume. Algebraically this reduces to a simpler equivalent form, Volume = (pi x Width x Depth x FillHeight) / 4, which is the same calculation as the area of a half-ellipse (using half the width and the full depth as the two semi-axes) times the fill height. All three inputs are entered in inches and divided by 12 before the math runs, and the final cubic-foot figure is divided by 27 to get cubic yards, then multiplied by 1.4 to approximate weight in tons.

Examples

Standard prefabricated well

A well measuring 39 inches wide and 16 inches deep (front to back), filled to 12 inches, converts to 3.25 ft, 1.33 ft, and 1 ft. That comes out to about 3.4 cubic feet, or roughly 0.126 cubic yards, weighing approximately 0.18 tons.

Larger custom well

For a wider well, 48 inches across and 24 inches deep, filled to 18 inches, the dimensions convert to 4 ft, 2 ft, and 1.5 ft. The formula returns about 9.4 cubic feet of gravel, which is roughly 0.349 cubic yards and weighs approximately 0.49 tons.

Advantages

Accounts for the curved shape of a typical window well instead of overestimating with a simple rectangular box calculation
Converts inch-based well measurements directly into an order-ready quantity in both cubic yards and tons
Lets a user quickly compare fill quantities for different fill heights without needing to redo geometry by hand

Common Mistakes

Measuring the well's depth as its vertical height above the basement floor instead of the front-to-back distance the well projects from the foundation wall
Filling gravel all the way up to the window frame rather than leaving the recommended few inches of clearance below the sill
Assuming a straight rectangular box formula applies, which overstates volume for a curved well and understates it for a squared-off one

Edge Cases to Watch For

A depth entered as 0 causes a divide-by-zero in the underlying scaling step, so the result becomes invalid; the depth field needs a real positive measurement to produce a usable answer.
The calculation assumes a curved, semicircle-style well; if the actual well is a straight-sided rectangular steel or plastic unit, the true volume will be somewhat higher than this estimate because a rectangular footprint holds more gravel than a curved one of the same width and depth.
The 1.4 tons-per-cubic-yard figure is a general approximation for drainage gravel; actual weight varies with the specific stone size and moisture content, so it should be treated as a rough ordering guide rather than an exact delivery weight.
The tool calculates volume for whatever fill height is entered but does not check it against the window sill height, so it is up to the user to leave the recommended clearance below the window rather than filling to the brim.

Common Use Cases

Homeowners replacing compacted dirt in a window well with drainage gravel to reduce water pooling against the foundation
Contractors or landscapers estimating gravel quantities across several window wells on a single job before ordering material
DIY basement waterproofing projects where a well needs to be topped off or refreshed with new drainage stone

Frequently asked questions

Why do window wells need gravel instead of just being left as bare dirt?

Bare soil in a window well compacts and doesn't drain well, allowing rainwater to pool against the foundation wall and potentially seep through the window or foundation - a gravel-filled well with a drain (in wells more than a couple feet deep) allows water to percolate down and away rather than accumulating, protecting both the window and the foundation.

Conclusion

By converting a window well's width, depth, and target fill height into a curved-footprint volume, this calculator gives a practical starting estimate for gravel orders in both cubic yards and tons. Because it approximates a semicircular or half-elliptical shape, results are most accurate for rounded wells and should be adjusted upward for square-cornered rectangular units.