Lot size to acres calculator
A lot 218 ft deep and 100 ft wide is 21,800 square feet, which is 0.5005 acres — just over the half acre a listing would call it. Lot size converts to acreage by dividing the square footage by 43,560, and the calculator above does it for trapezoidal and four-sided lots too, where frontage and rear boundary differ.
Your answer
Lot size0.4132 ac
- In square feet
- 18,000 ft²
Plot sheet
What this means for the field in front of you:
- This plot in every unit
- 1,672 m² · 16.72 a · 0.1672 ha · 18,000 ft² · 0.4132 ac
- As the register writes it (ha a ca)
- 0 ha 16 a 72 ca
- Distance around the plot
- 164.6 m
- Measured by
- measured sides
Three fields are kept; saving a fourth replaces the oldest.
How the answer is worked out
Residential lots are sold in square feet and regulated in acres, so the same piece of ground gets described two ways in one transaction. This page converts between them from the dimensions on the plat rather than from a rounded figure in a listing.
Rectangular lot. Frontage × depth, in feet, divided by 43,560.
Lot that narrows to the rear. Pick the trapezoid: the two parallel boundaries and the perpendicular depth between them. The area is the average of the two parallel sides times the depth — noticeably less than frontage × depth on a wedge-shaped lot, and that difference is often the difference between meeting a minimum lot size and not.
Irregular four-sided lot. Four boundary lengths plus one diagonal from the plat. The diagonal is what makes the shape measurable at all: four sides alone hinge, and any tool that answers without one is guessing.
Corner lot on a curve. Neither this nor any other simple calculator handles a curved boundary honestly. Take the area from the plat instead, or measure the straight-line ring on the mapping page, which computes the polygon from the corner coordinates.
The answer is printed to four decimal places of an acre because that is the precision at which a “half-acre lot” stops being half an acre: 0.5005 is not 0.5000, and on a lot priced per acre the gap is real money. The copy button gives the unrounded number for a contract or a spreadsheet, and the sheet prints the same lot in square feet, square metres, ares and hectares for anyone on the other side of the deal who does not think in acres.
A worked example
Every figure below is computed by the calculator above from this case — nothing here is typed in by hand:
- Shape of the lot
- Rectangle
- Lot measured in
- ft
- Lot depth (front to back)
- 218
- Lot frontage (along the road)
- 100
Lot size:0.5005 ac
- In square feet
- 21,800 ft²
- This plot in every unit
- 2,025 m² · 20.25 a · 0.2025 ha · 21,800 ft² · 0.5005 ac
- As the register writes it (ha a ca)
- 0 ha 20 a 25 ca
Worked by Agrimetra with the same engine the calculator above runs.
What this means in the United States
Residential listings give lot size in square feet, while zoning, septic permits and property tax classes are written in acres, so buyers convert the same number twice. The threshold that matters is set county by county — check the local ordinance before treating an acreage figure as a permit.
Questions growers actually ask
- How many square feet is a half-acre lot?
- 21,780 square feet exactly. Anything in the neighbourhood of that gets called a half acre in a listing, so a lot advertised that way can be several hundred square feet either side of it — which matters when a setback or a minimum lot size is written in acres.
- My lot is wider at the road than at the back. What do I use?
- The trapezoid shape: the frontage, the rear boundary and the depth between them. Multiplying frontage by depth would charge you for land that is not there. If the lot is a true quadrilateral with no parallel sides, use the four-sided shape and add the diagonal from the plat.
- Is this the figure the assessor uses?
- Not necessarily. Recorded acreage comes from the plat and the legal description, and it can differ from what the dimensions multiply out to. Use this to check a listing and to plan; use the recorded figure for anything legal.
- Does lot size decide what I can build?
- Only together with the local ordinance. Minimum lot size, setbacks, coverage and septic rules are set county by county and are frequently written in acres while the listing is in square feet — which is exactly where this conversion is needed, and exactly where it stops being the whole answer.
Last updated:
Sources
- NIST Handbook 44 — general tables of units of measurement (checked September 22, 2026)
- NIST — the US survey foot is deprecated as of 1 January 2023 (checked September 22, 2026)