Lot Size and Coverage Calculator

What this calculator does

Lot dimensions, setbacks on all four sides, a proposed footprint and a coverage limit go in. It returns the lot in square feet, acres, hectares and square metres, the perimeter, the buildable envelope after setbacks, the coverage percentage your footprint produces, the maximum footprint your rules allow, and whether the building actually fits.

The gap between “lot size” and “what you can build on it” is usually much larger than people expect, and that gap is the whole reason this page exists.

43,560

An acre is 43,560 square feet.

There is no way to derive that from anything. An acre was historically the area a yoke of oxen could plough in a day, later standardised as one chain (66 feet) by one furlong (660 feet) — which multiplies out to 43,560 and has stayed there ever since.

Some anchors worth carrying:

• A quarter-acre lot is 10,890 sq ft — roughly 100 × 109 feet.
• A half-acre is 21,780 sq ft.
• An American football field including end zones is about 1.32 acres.
• A hectare is 2.471 acres, or 107,639 sq ft.

The buildable envelope is not the lot

This is the number that surprises people, and it is worth doing the arithmetic explicitly.

Take a 150 × 80 foot lot — 12,000 square feet, a shade over a quarter acre. Reasonable suburban setbacks: 25 feet front, 20 feet rear, 7.5 feet each side.

• Buildable depth: 150 − 25 − 20 = 105 feet
• Buildable width: 80 − 7.5 − 7.5 = 65 feet
• Buildable envelope: 6,825 square feet

You have lost 43 percent of the lot before drawing anything. And that is the envelope, not the permitted footprint — the coverage limit then applies on top of it.

Which is why two lots of identical acreage can support very different houses. A narrow deep lot loses proportionally far more to side setbacks than a wide shallow one, because the side setback is subtracted twice from the smaller dimension.

Coverage limits, and what counts

Most zoning ordinances cap the share of the lot a building may occupy — commonly 30 to 40 percent in suburban residential zones, less in low-density zones, more in urban ones.

The trap is in the definition, which varies:

• Some ordinances count only the principal building footprint.
• Many add accessory structures — garages, sheds, pool houses.
• Many count driveways, patios and walkways.
• Some regulate impervious surface instead, which includes everything water cannot pass through and is a much broader category.

That last one catches people badly. A house well inside a 35 percent building coverage limit can breach a 50 percent impervious surface limit once the driveway, patio and pool deck are counted — and the fix at that stage is expensive permeable paving or a smaller driveway.

Read your own ordinance’s definition before trusting any coverage number, including this one.

Property lines are not where you think

Two recurring errors, both expensive.

The front line is usually not the kerb. It generally sits at the edge of the public right-of-way, which often extends several feet — sometimes ten or more — into what you mow and maintain. Measure from the kerb and your entire building envelope shifts toward the street, which is the direction that fails an inspection.

Fences and hedges are not property lines. They were placed by somebody, at some point, approximately. Long-standing encroachments are common, and in some jurisdictions they create legal complications of their own.

For anything you are going to build against, get a survey. It is a modest cost relative to moving a foundation, and it is the only document that settles the question.

Irregular lots

The rectangular calculation here is exact for a rectangle and approximate for anything else.

For irregular lots, break the shape into rectangles and triangles, calculate each, and add them. A triangle is half the base times the height.

Corner lots deserve a specific warning: many ordinances treat both street frontages as fronts, applying the larger front setback to two sides instead of one. A corner lot can therefore have a substantially smaller buildable envelope than an identical interior lot, which is the opposite of most people’s intuition about corner lots being desirable.

Pie-shaped lots on cul-de-sacs have a narrow frontage where the side setbacks bite hardest, and the recorded area often overstates how usable the lot is.

What else constrains the envelope

Setbacks and coverage are the two most common constraints and not the only ones. Before assuming a footprint works, check:

• Height limits and, in many places, daylight planes that slope the allowable envelope inward as it rises
• Floor area ratio (FAR), which caps total floor area across all storeys — a separate limit from footprint coverage
• Easements for utilities, drainage or access, which are usually unbuildable regardless of setbacks
• Wetlands, floodplains and steep-slope overlays
• Septic system requirements, which on unsewered lots often govern everything else
• Homeowner association rules, which are frequently stricter than zoning and are enforced privately

What this leaves out

• Irregular geometry, as above.
• Height, FAR and daylight planes.
• Easements and encumbrances.
• Corner-lot double frontage rules.
• Variances, which sometimes make an impossible layout possible.
• Topography. A flat 6,825 square feet and a steeply sloping 6,825 square feet are very different building propositions — the excavation calculator covers what moving the difference costs.

Once the footprint is settled, the building cost calculator turns square footage into a budget, and the home affordability calculator works from the other end.