How this instrument works
A hectare is simply the square hectometre: 100 m by 100 m, or 10 000 m². Its ancestor is the are, 100 m², created by French law in 1795 when surveyors wanted land measures that sat neatly inside their new decimal system. The are itself has almost vanished from use, but its hundredfold survived, because it happens to match how farmland is actually parcelled. BIPM lists the hectare among non-SI units accepted for use alongside SI, and one square kilometre holds exactly 100 of them.
An acre was never designed; it accumulated. Medieval English practice took it as roughly what one ploughman and two oxen could turn over between sunrise and sunset, and its shape still records that origin — one chain wide by one furlong long, 22 yards by 220. Furlong means furrow long: as far as oxen pulled before turning paid for itself. Edmund Gunter's 66-foot chain of 1620 nailed down the width, leaving 4840 square yards, or 43 560 square feet, per acre.
Those square yards became exact metres in 1959, when six English-speaking standards bodies fixed the yard at 0.9144 m. Squaring gives 0.83612736 m² per square yard, and 4840 of those make an acre of 4046.8564224 m² precisely. Divide 10 000 by that figure and you get 2.4710538146716…, a ratio that is perfectly rational yet never terminates as a decimal. Land registers, forestry inventories, and European farm-support claims run on hectares; deeds in the United States and sale boards across Britain and Ireland still speak acres.
- Type your parcel size into the Hectares (ha) field — 5 ha is preloaded as your starting point.
- Read the answer on the Acres (ac) line, which recalculates with every keystroke; there is nothing to submit.
- Small plots are welcome: that field steps in thousandths, so a 0.125 ha building plot reads across without fuss.
- Going backwards, from acres, divide your figure by 2.47105381467 — or nudge hectare values until Acres (ac) lands on your number.
- Choose your rounding with care: one hundredth of an acre is about 40 m², which flatters paddocks but ruins boundaries.
Worked example — five hectares on a sale board
A land agent holds a field entered on the register as 5.000 ha and needs an acre figure for printed particulars. Enter 5 into Hectares (ha) and Acres (ac) returns 12.3552690734. Sale boards will read 12.36 acres, while contract plans keep hectares, because that is what registers actually record.
Verify it without the multiplier and every digit survives: five hectares is 50 000 m², and 50 000 ÷ 4046.8564224 gives 12.35526907… again. Rounded hard to whole acres you would print 12, losing 1438 m² of grazing. At 12.36 acres buyers sit within 20 m² of truth — fine for grazing, useless in boundary disputes.
Questions
Is one hectare exactly 2.47105381467 acres?
No, that decimal is rounded, though both units behind it are exact. One hectare is 10 000 m² and one acre 4046.8564224 m², yet dividing either by the other yields 2.4710538146716533…, which runs on without repeating soon enough to write down. Twelve significant figures leave an error under one part per trillion — about one hundredth of a square millimetre across the whole hectare, so it can never matter for land. For arithmetic that must close exactly, divide square metres by 4046.8564224 instead of multiplying by any decimal.
Is an American acre the same size as a British acre?
Yes, exactly the same, which makes the acre unusual. Since 1959 both descend from an international yard of 0.9144 m, so 4840 square yards means 4046.8564224 m² on either side of that ocean. Compare gallons, pints, tons, or fluid ounces, where imperial and US customary values differ enough to spoil recipes and tanker loads. Only the US survey acre, built on an older foot, breaks that agreement.
What is a US survey acre, and when does it matter?
It is the acre built on the survey foot of 1200⁄3937 m rather than the international foot, giving 4046.87261 m² — larger by about four parts per million, or 0.016 m² per acre, roughly postcard-sized. That gap only surfaces in Public Land Survey System descriptions and older state plane coordinates, where it compounds across townships into real metres. NIST and NOAA retired the survey foot at the close of 2022, so anything new, including figures here, uses the international acre.
Why is an acre such an awkward number of square metres?
Because nobody chose it in metres. An acre is one chain by one furlong: 22 yards by 220, or 4840 square yards, shaped ten to one by ploughing long furrows and turning oxen as rarely as possible. Multiply 4840 by 0.83612736, one squared yard in square metres, and 4046.8564224 falls out. Untidy metrically, tidy in its own system — ten square chains per acre, 640 acres per square mile, 40 acres per PLSS quarter-quarter.
How many hectares fit in a square kilometre?
Exactly one hundred. Square kilometres span 1000 m each way, hectares 100 m each way, so one hundred tile inside. One common slip treats hectares as metric acres and square kilometres as metric square miles: the square mile is 258.999 ha, not 100. Another confusion is the acre-foot, which measures water volume — one acre flooded one foot deep, about 1233.5 m³ — and has nothing to do with area at all.
Which countries actually work in hectares versus acres?
Hectares dominate national cadastres, forestry inventories, FAO statistics, and European farm-support claims, where payment per hectare makes precision financial. Acres persist across United States deeds and survey descriptions, and in British and Irish farm sales, where fields held on the register in hectares are still advertised in acres, because buyers picture acres. India, Pakistan, and rural Canada mix both with bigha, kanal, guntha, and arpent, so ask which measure any quoted figure came from before trusting it.