How this instrument works
The acre began as work rather than geometry: one chain by one furlong, 66 feet by 660 feet, roughly what a man with an ox team could turn over between sunrise and dusk. Edmund Gunter's surveying chain of 1620 fixed those dimensions in brass, and ten square chains have meant one acre ever since — 43,560 square feet, or 4,840 square yards. Nothing about that total is round, because nothing about it was designed to be.
The hectare came from the opposite instinct. French metric nomenclature of 1795 named the are, an area of 100 square metres, and a hectare is simply a hundred of them: 10,000 square metres. It sits outside the SI proper yet is accepted for use alongside it, which is why forestry inventories, European subsidy claims and national cadastres all count land in hectares while auction particulars in Kentucky or County Cork still shout acres.
Bolting the two together took a treaty. The 1959 international yard and pound agreement set the yard at exactly 0.9144 metre, so a square yard is exactly 0.83612736 square metre, and 4,840 of those come to 4046.8564224 square metres. Divide by 10,000 and out falls 0.40468564224 — a terminating decimal, exact to its last digit, which is why this sheet carries all eleven places instead of the usual 0.4047.
- Type your parcel size into the Acres (ac) field; 10 acres is loaded as a starting point.
- Read the answer from Hectares (ha) — it recomputes on every keystroke, with no button to press.
- The result shows six decimals. For deeds and subsidy forms, round at the very end, never partway through.
- Going the other way? Divide your hectare figure by 0.40468564224, or multiply it by 2.4710538147.
Worked example — ordering seed for ten acres
A grower has a 10-acre block and a seed bag rated in kilograms per hectare: 180 kg/ha. Enter 10 in Acres (ac) and Hectares (ha) reads 4.046856, the six-decimal face of the exact value 4.0468564224. Multiply through and the order comes to 728.43 kg, so 730 kg covers the block with a little in hand — whereas calling the area a flat 4 ha would have left the drill eight kilograms short.
The same factor holds at any scale, because it terminates. A quarter-section homestead of 160 acres is 64.7497027584 hectares exactly; a full 640-acre section of the US Public Land Survey is 258.9988110336 hectares. Nothing drifts along the way — every digit traces back to a yard of 0.9144 metre and to nothing else.
Questions
Is one acre exactly 0.40468564224 hectares?
Yes, exactly — this is arithmetic, not measurement. An acre is defined as 4,840 square yards, the yard has been exactly 0.9144 metre since the 1959 international yard and pound agreement, and a hectare is exactly 10,000 square metres. Chain those definitions together and you get 4046.8564224 divided by 10,000, with no uncertainty attached anywhere. Because 0.9144 squared terminates, so does the factor: eleven decimal places and then silence.
How many acres make one hectare?
About 2.4710538147, and that direction is the untidy one. Inverting an exact terminating decimal seldom yields another: 1 ÷ 0.40468564224 opens out to 2.471053814671653… and keeps going. The quotient remains a plain ratio of whole numbers all the same, so precision survives intact — keep 2.471 for conversation and let the machine hold the tail for paperwork. Picture a hectare as a square 100 metres a side, near enough forty per cent roomier than a full-size association football pitch.
Does an American acre differ from a British one?
Not today — both are the international acre of 4046.8564224 square metres. Two caveats survive. Survey records drawn in the United States before 2023 may rest on the US survey acre, built from a survey foot of 1200⁄3937 metre and about four parts per million larger; across a 640-acre section that gap is some ten square metres, and NIST and NOAA retired the unit at the end of 2022. Older Irish and Scottish acres are another matter entirely: the Irish acre ran to 7,840 square yards, over sixty per cent bigger than the one this page converts.
Why is an acre 43,560 square feet rather than a round figure?
Because it describes a shape of labour. An acre was one furlong long by one chain wide — 660 feet by 66 feet — a furlong being a furrow's length, as far as an ox team ploughed before turning. Multiply and 43,560 square feet appears. Gunter's 66-foot chain, cut into 100 links, kept the number circulating among surveyors: ten square chains, one acre. That ungainly total is a fossil of medieval fieldwork rather than any decision by a standards body.
Can I safely round the factor to 0.4047?
For conversation yes, for registered boundaries no. Rounding up to 0.4047 is high by about thirty-five parts per million, so a 100-acre farm reads as 40.47 hectares instead of 40.468564 — fourteen square metres of land that is not there. Harmless in a sale advert, awkward in a subsidy claim paid per hectare, unacceptable in a cadastral plan. Round the answer for display; leave the factor alone until after you multiply.
How do I handle a deed written in acres, roods and perches?
Reduce it to decimal acres first. Four roods make an acre and forty perches make a rood, giving 160 perches to an acre. A parcel recorded as 3a 2r 12p works out at 3 + 0.5 + 0.075 = 3.575 acres, which this sheet turns into 1.446751 hectares. Nineteenth-century conveyances across England, Ireland and the colonies lean on that notation constantly, and the perch column is where transcription errors usually hide.