# Old Irish units of measurement

In old Irish law, a wide variety of measurements were used.

## 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 25 cm (10 inches).

Table of length units
Unit Meaning Relative
value
SI
value
Imperial
value
Notes
grain 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
eggshell 1 55 ml 2 fl. oz.
méisrin 12 660 ml 1 pint 3 fl. oz.

## Mass

Table of mass units
Unit Meaning Relative
value
SI
value
Imperial
value
Notes
grain 1/8 0.05 g 0.772 Gr One grain of the best wheat.
pinginn penny 1 0.4 g 6.173 Gr
screpall scruple 3 1.2 g 18.519 Gr
unga 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.