How to use the result
The calculator sizes a debris or weather cover from the water shape plus overhang. It does not design a certified safety cover system. If fall protection is required, use a cover rated and installed for that purpose.
Anchor line beats waterline
If your straps or water bags sit farther out on the deck, increase the overhang so the cover reaches the real anchor line without stretching the grommets sideways.
How each shape is handled
Rectangular pools are the straightforward case: length plus twice the overhang, width plus twice the overhang. Round pools are calculated as a square whose side equals the diameter plus twice the overhang, because rectangular stock tarps are what most people actually buy — you end up with four corners of excess that get folded or trimmed. Oval pools use the maximum length and maximum width of the waterline, which means the tool is sizing the bounding rectangle, not the oval itself.
That bounding-rectangle behavior is deliberate, but it means the area figure is larger than the water surface for anything that is not a rectangle. Use the dimensions for buying and treat the area as an upper bound.
Choosing an overhang
Overhang is not decoration — it is what gives you something to anchor. Too little and the cover pulls at the grommets from a sideways angle every time the wind lifts it. Common starting points:
| Situation | Typical overhang per side | Why |
|---|---|---|
| Debris cover, sheltered yard | 1–2 ft | Enough to reach water bags or clips on the deck |
| Winter cover, open exposure | 2–3 ft | Wind lift needs a wider anchor base |
| Above-ground pool | 2–4 ft | Cover must reach a cable or strap below the top rail |
Measure to where the straps, water bags, or anchor points actually sit — not to the water edge. If your anchors are 30 in out on the deck, an 18 in overhang will not reach them no matter how large the tarp looks on paper.
Plan for the water that lands on top
A flat cover over an open pool becomes a basin. Rain and snowmelt pool in the center, and the weight can be substantial — water runs about 8.3 lb per gallon, so even a shallow pool of standing water on a 16 × 32 ft cover is measured in hundreds of pounds. That load pulls the cover inward, drags the edges off their anchors, and can submerge the whole thing.
The usual answers are a cover pump, an air pillow underneath to create a crown that sheds toward the edges, or a support system that keeps a slope. Decide which one you are using before you buy the cover, because it changes how much slack you want.
This is a debris and weather cover, not a safety cover
A tarp sized by this tool is not rated to hold the weight of a person, a child, or a pet, and it should never be relied on as fall protection. Certified safety covers are engineered products with tested load ratings, anchored into the deck, and installed to a specification. If anyone could reach the pool unsupervised, that is the product you need — and local codes may require barriers regardless of what cover you use.