Methodology

Sources reviewed May 2026

How we build and maintain AcresToSquareFeet.com: the primary sources we anchor against, what is in scope and out of scope, the explicit calculation formulas behind every conversion, the monthly refresh cadence, and the limitations we acknowledge. For the shorter editorial framing, see /about.

Primary sources

Source	Role	What we take from it	Refresh
NIST and the 1959 International Yard and Pound Agreement	Primary: international acre definition	1 international foot = 0.3048 m exactly; 1 international yard = 0.9144 m exactly; 1 international acre = 43,560 sq ft exactly = 4,046.8564224 m2 exactly. The IYPA was a joint agreement of the US, UK, Canada, Australia, New Zealand, and South Africa that fixed the foot and yard at the metric values above and superseded the previously independent national foot definitions. Source: NIST Federal Register notices implementing the 1959 IYPA.	On NIST or IYPA revision
NIST Handbook 44 and NIST SP 811	Primary: US Customary units	Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices (Handbook 44) and the Guide for the Use of the International System of Units (SP 811). Used as the canonical US reference for the international foot, the international acre, and the square-foot / square-yard relationships.	On NIST revision
BIPM SI Brochure	Primary: metric area definitions	Square metre as the SI derived unit of area; hectare = 10,000 m2 (accepted for use with the SI but not part of the SI). Source: Bureau International des Poids et Mesures, SI Brochure, 9th edition.	On BIPM revision
National Geodetic Survey and the 2022 US Survey foot deprecation	Primary: US Survey vs International foot	The US Survey foot was defined as exactly 1200/3937 m, producing an acre approximately 43,560.1742 sq ft and 4,046.872609874 m2 (about 4 ppm larger than the international acre, or 3.22 sq ft per 640 acres). Effective October 1, 2022, the NGS in joint Federal Register Final Notice with NIST deprecated the US Survey foot for new federal surveys; SPCS 2022 state-plane coordinate systems use the international foot.	On NGS revision
US Geological Survey (USGS)	Primary: canonical US land area	USGS publishes authoritative US land and water area datasets and operates the National Map. Used as the source-of-record for state and county area figures and for sanity-checking PLSS section computations.	Annual cross-check
USDA NASS Land Values, Cash Rents, and Crop Production Summary	Primary: farmland pricing and yield per acre	Annual USDA National Agricultural Statistics Service publications. 2024 figures: US average corn yield 183.1 bu/ac; soybean 51.4 bu/ac; winter wheat 51.2 bu/ac; national cropland cash rent $155/ac (Iowa $268/ac); national pasture rent $14.50/ac. State-by-state farmland value averages from the Land Values publication. Source: nass.usda.gov.	Annual (Crop Production Summary January; Land Values August)
USDA NRCS Natural Resources Inventory	Secondary: farmland classification	Natural Resources Conservation Service Natural Resources Inventory: prime farmland classification, irrigated vs non-irrigated, soil type by region. Cross-referenced where state-by-state farmland figures need land-classification context.	Per NRI release cycle
US Bureau of Land Management (BLM) Public Land Survey System	Primary: PLSS section and township geometry	640 acres = 1 square mile = 1 PLSS section; 36 sections in a 6-mile by 6-mile township (23,040 ac total); 160 ac = 1 quarter-section (the Homestead Act 1862 grant); 40 ac = 1 quarter-quarter-section. PLSS covers 30 western US states (states east of Pennsylvania and Ohio use metes-and-bounds). Source: BLM Cadastral Survey manuals.	On BLM manual revision
FAA airport-acre conventions	Secondary: airport area context	Federal Aviation Administration conventions for general-aviation vs commercial-service airport sizing in acres. Used where landmark-comparison references include airports.	Per FAA Airport Master Record cycle
US Census Bureau residential lot data	Primary: residential lot size norms	American Housing Survey and the Survey of Construction publish residential lot-size statistics by US state. US median residential lot approximately 0.19 acres (8,276 sq ft). Used for the by-state residential parcel norms.	Per Census publication cycle (American Housing Survey biennial)
UK Weights and Measures Act 1985	Primary: UK Imperial acre	The UK Imperial acre is defined as 4,840 sq yd = 43,560 sq ft. Post-1959 IYPA the UK Imperial acre is numerically identical to the international acre because UK statute adopted the international foot of 0.3048 m exactly. Pre-1959 the UK foot was defined via the British Imperial Yard standard, marginally different by a sub-ppm.	On UK statute revision
UK Ordnance Survey	Primary: UK land measurement context	The Ordnance Survey is the UK national mapping agency. Operates the OS National Grid (Easting / Northing in metres), publishes UK land area and land-use data. Used as the UK counterpart to USGS where the site references UK land context.	Per OS data release cycle
Federal Register Final Notice (NGS / NIST 2022)	Primary: the 2022 deprecation paperwork	The joint NGS / NIST Federal Register Final Notice formally deprecating the US Survey foot effective October 1, 2022. Required reading for anyone who needs to know whether a federal land record uses the international foot or the legacy survey foot.	Static (one-off notice)
ESRI / SPCS 2022 reference materials	Secondary: state plane coordinate systems	Reference for the new SPCS 2022 state plane coordinate system that replaces SPCS 83 and adopts the international foot. Used to give modern GIS context to the survey-foot deprecation discussion.	Per ESRI SPCS 2022 publication updates

In scope

International acre (43,560 sq ft exactly per 1959 IYPA, NIST-anchored)
US Survey acre (43,560.174 sq ft, 4 ppm gap, NGS-deprecated October 2022)
UK Imperial acre (4,840 sq yd, BSI / UK Weights and Measures Act 1985)
Chain and furlong derivation (Gunter chain 1620)
PLSS section (640 ac), quarter-section (160 ac), township (23,040 ac), range, principal meridian
USDA NASS state-by-state farmland prices and cash rent (2024)
USDA NASS crop yield per acre (corn 183.1 bu/ac, soybean 51.4 bu/ac, winter wheat 51.2 bu/ac, 2024)
US Census Bureau residential lot size norms (US median ~0.19 ac)
Decimal-acre fractions (0.05 ac through 640 ac) on the conversion-table page
Reverse-direction sq ft to acres (with common lot sizes 2,000 to 435,600 sq ft)

Out of scope

Individual property surveys (retain a licensed land surveyor in your state)
Legal title disputes, boundary work, and metes-and-bounds adjudication (retain a surveyor and an attorney)
Mineral-rights and surface-rights conveyances (different legal regime)
Real-time agricultural production planning beyond cited USDA averages
Sub-foot precision survey work (refer to NGS, state geodetic agencies, or licensed surveyors)
Modern GIS coordinate transformations (refer to SPCS 2022 / state-plane and ESRI / Esri documentation)

Calculation framework

Acre to square feet

sq ft = acre x 43,560 (international acre, exact per 1959 IYPA)

1 ac = 43,560 sq ft. 2.5 ac x 43,560 = 108,900 sq ft.

Acre to square metres

m2 = acre x 4,046.8564224 (exact derived from 1 ft = 0.3048 m)

1 ac = 4,046.8564224 m2 (we display 4,046.86 in body text). 10 ac = 40,468.56 m2.

Acre to hectare

ha = acre x 0.40468564224 (exact)

1 ac = 0.40469 ha (we display 0.4047 in body text). 100 ac = 40.469 ha.

Acre to square mile

sq mi = acre / 640 (1 PLSS section = 640 ac exactly)

640 ac = 1 sq mi. 23,040 ac = 36 sq mi = 1 PLSS township.

US Survey acre vs international acre

1 US Survey ac = 43,560 x (1200/3937)^2 m2 = 4,046.872609874 m2

About 4 ppm larger than the international acre. 640 US Survey acres = 640 international acres + ~3.22 sq ft.

Chain and furlong derivation

acre = 1 Gunter's chain x 1 furlong = 66 ft x 660 ft = 43,560 sq ft

The Gunter chain (1620) was 22 yards = 66 ft. The furlong was 1/8 mile = 220 yards = 660 ft. Chain x furlong = 22 x 220 = 4,840 sq yd = 43,560 sq ft.

PLSS section, township, range

Section = 640 ac = 1 sq mi. Township = 36 sections in a 6 x 6 mile grid = 23,040 ac. Quarter-section = 160 ac (Homestead Act 1862).

A typical Midwest legal description like 'NE 1/4 Sec 16, T 1 N, R 1 E' refers to the 160-acre northeast quarter of section 16 in township 1 north, range 1 east of a named principal meridian.

$ per acre to $ per sq ft

$/sq ft = ($/acre) / 43,560

$10,000/ac = $0.2296/sq ft. $50,000/ac (urban lot) = $1.148/sq ft. Useful for comparing rural parcels priced per acre against urban lots priced per sq ft.

Refresh cadence

Monthly first-business-week pass driven by a single LAST_VERIFIED_DATE constant in the codebase. The constant drives the footer date, the site-wide WebSite JSON-LD dateModified, every per-page Article schema dateModified, and the "Reviewed May 2026" badges on this page and /about. Out-of-cycle refresh triggers:

NIST or IYPA standards revision (international foot or yard redefinition)
BIPM SI Brochure update
NGS Federal Register Final Notice (additional Survey-foot or geodetic system updates)
USDA NASS Crop Production Summary (annual January) and Land Values (annual August)
USGS land-area dataset revision
BLM Cadastral Survey manual revision
UK Ordnance Survey reference data update
Reader-flagged correction with cited counter-source

Limitations

International acre and US Survey acre are numerically distinct but differ by only 4 ppm (3.22 sq ft per 640 ac). For residential transactions this is negligible; for federal land records pre-October 2022 it matters and we flag it on every relevant page.
The commercial acre (~36,000 sq ft) and builder's acre (~40,000 sq ft) are informal industry conventions, not legal units. We document them so readers recognise them in marketing copy but we do not treat them as standards.
PLSS coverage is primarily the 30 states surveyed under the federal land system; states east of Pennsylvania and Ohio (including the original 13 colonies) use metes-and-bounds descriptions. State-by-state context reflects this distinction.
USDA NASS state averages are means, not medians, and can be skewed by very large or very small operations. We cite the mean because that is the published statistic and note the limitation where it matters (Texas average farm 1,379 ac hides 250,000-ac ranches at one end and 50-ac smallholdings at the other).
FAA airport-acre conventions vary by airport class and the regulatory definition (Part 139 vs general aviation); we cite typical values rather than per-airport actuals.
Real-world landmark areas (Vatican City, Central Park, Disneyland, Walmart Supercenter average) are verified against operator and government published data but represent point-in-time figures; large operators occasionally redevelop and acreage shifts.

Corrections

For corrections, source disagreements, or new primary sources we should be tracking, email us via the contact form at digitalsignet.com. Cite the page URL, the specific figure or claim, the source you are challenging it against, and the source URL. We respond within 5 business days. Substantive corrections trigger an out-of-cycle LAST_VERIFIED_DATE roll.

This site is reference and educational. It is not a property survey, a legal land description, or an appraisal. For legal land descriptions and boundary work, retain a licensed land surveyor in your state. For land valuation (purchase, sale, financing, tax assessment), retain a licensed appraiser or your local assessor.