Old Irish units of measurement

Early Irish law texts record a wide variety of units of measurement, organised into various systems.

Length

A troighid ("foot") was the length of a man's foot, divided into twelve ordlach, "thumb-lengths". These figures assume a man's foot to measure 10 in (25 cm).

Table of length units
Unit	Meaning	Relative value	SI value	Imperial value	Notes
grán	grain	^1⁄36	0.7 cm	1/4 inch
ordlach	thumb-length	^1⁄12	2.1 cm	0.8 in
bas	palm	^1⁄3	8.4 cm	3.3 in
dorn	fist	^5⁄12 or  ^1⁄2	10.4 or 12.5 cm	4 or 5 in
troighid	foot	1	25 cm	9.9 in
céim	step	2.5	62.5 cm	2 ft 1 in
deiscéim	double-step	6	1.5 m	4 ft 11 in
fertach	rod	12	3 m	9 ft 10 in
forrach		144	36 m	39.4 yards

A magh-space was a unit set at the distance from which a cock-crow or bell could be heard. Other units such as inntrit and lait appear in documents; their value is uncertain, perhaps being equivalent to 1 and 2 fertachs respectively.

Area

The basic unit of area was the tir-cumaile, "land of three cows", as it was an area of land that was at some point worth three cows. It is sometimes erroneously interpreted as the area needed to graze three cows, but it is far too large for that; in modern Ireland, a cow grazes on about 0.4 ha, so twenty or more could graze a tir-cumaile. Ireland in total covered about 870,000 tir-cumaile.

Table of area units
Unit	Meaning	Relative value	SI value	Imperial value	Notes
tir-cumaile	land of three cows	n/a	9.3 ha	23 ac	72 square forraigh
achar	acre	1			Adopted following Norman invasions.
seisrech	plough-land	120
baile	townland	1,440			The term "townland" is still used to designate very small named areas in the countryside.
tuath		4,320			Originally a term for a petty kingdom.

Capacity

A hen's eggshell was used as a standard unit, roughly 55 ml.

Table of capacity units
Unit	Meaning	Relative value	SI value	Imperial value	Notes
og	eggshell	^1⁄12	55 ml	2 fl. oz.
méisrin	little measure	1	660 ml	1 pint 3 fl. oz.
olderb	great vessel	12	7.92 L	1.74 gallons
oilmedach	great mead [vessel]	144	95 L	21 gallons

Mass

Table of mass units
Unit	Meaning	Relative value	SI value	Imperial value	Notes
grán	grain	^1⁄8	0.05 g	0.772 Gr	One grain of the best wheat.
pinginn	pennyweight	1	0.4 g	6.173 Gr
screpall	scruple	3	1.2 g	18.519 Gr
ungae, uinge	ounce	72	28 g	1 oz	Word borrowed from the Latin uncia, although an older native name was mann.

Time

A night (oídhche) was used as a measure for time, in preference to a day. Unlike most traditional societies, the Irish held that a new day began at sunset, not at sunrise, so that a Wednesday night would precede the day of Wednesday.

See also

• List of obsolete units of measurement
• Metrication in Ireland

References

Sources

• Joyce, P. W. (1906). "23: Measures, Weights, Mediums of Exchange". A Smaller Social History of Ancient Ireland. Retrieved 20 May 2015.
• "tir cumaile [Old Irish]". sizes.com. Sizes, Inc. 1 September 2008. Retrieved 20 May 2015.
• Seebohm, Frederic; Hugh Exton, Seebohm (1914). "2: The Irish units of tribute and food-rent". Customary acres and their historical importance, being a series of unfinished essays". Longman's, Green, & co. Retrieved 20 May 2015.