Acres, hectares and the surveyor's chain — why land parcels don't round

Every land document larger than a suburban lot uses either the acre or the hectare, and every cross-border deed has a moment where those two numbers need to line up. The conversion factor — 1 ha = 2.47105381 acres — is exact, but where it comes from and why land is still measured in "chains" in some US counties is a longer story than the calculator alone tells.

The exact factors

1 acre = 43,560 sq ft = 4,046.8564224 m² (exact) > 1 hectare = 10,000 m² = 2.4710538147 acres > 1 square mile = 640 acres = 2.589988110336 km² > 1 chain = 66 ft = 20.1168 m; 1 acre = 10 sq chain

The 66-foot chain is the anchor of the entire US Public Land Survey System. Edmund Gunter designed it in 1620 so that ten square chains would equal exactly one acre — a decimal-friendly relationship inside an otherwise imperial system. Every township, section and quarter-section line in the American Midwest was originally paced out by two rod-men dragging a physical 66-foot chain across the prairie.

Where hectares came from

The metric system's designers wanted a land unit that scaled nicely: 1 are = 100 m², 1 hectare = 10,000 m² = 100 ares, 1 km² = 100 hectares. Farmers found the are too small (roughly a rugby try-zone) and the km² too large, so the hectare became the practical unit for fields, forestry parcels and cadastral maps almost everywhere outside the US and UK.

The US survey foot problem

Until 2022 the US had two definitions of the foot: the international foot (0.3048 m exactly) and the older US survey foot (1200/3937 m ≈ 0.30480061 m). The difference is 6.4 mm per 100 km — invisible on a house lot, but a full square metre on a section-line acre in Nebraska. NIST retired the US survey foot on 31 December 2022, but every legacy plat and every state-plane coordinate written before that date still uses it. When you see "US acre" instead of just "acre", the surveyor is warning you.

Working the conversion in practice

A 120-hectare Bavarian farm is being marketed to a Californian investor. Convert once: 120 × 2.47105 = 296.53 acres. Which is 296 acres and 21,400 sq ft, or in the older rural US phrasing, roughly "296 acres and half a lot". A single decimal error here — 296 vs 2,965 — is the difference between a farm and a subdivided ranch, so always sanity-check by comparing hectares to the m² figure the deed already lists.

Roofing, floor plans and the 'square'

Below the acre scale, the same units branch: builders in the US quote roofing in "squares" (1 square = 100 sq ft = 9.2903 m²), while European roofers quote m² directly. Real-estate listings mix things further — Australian house floor plans are often in m² and "squares" simultaneously, where 1 (Australian building) square = 100 sq ft. Always check which convention the listing is using.

Worked example — a 5-acre house lot

5 acres × 4,046.856 = 20,234.28 m² = 2.023 hectares. That is a square about 142 m on a side, or about 466 ft on a side — roughly one-and-a-half American football fields end-to-end. Land buyers who have only ever thought in city blocks systematically under-estimate acreage by 2–3× on their first purchase; converting to a mental picture (a football field is 1.32 acres) helps.

The five-second recap

One hectare is about two-and-a-half acres. One acre is 43,560 square feet. One square mile is 640 acres. If a deed says "US survey foot" or "US acre", track that flag through every downstream calculation — the 2 ppm difference is real.