The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 8, 'Dates, Numbers, Currency, and Weights and Measures'
8.5 Weights and Measures
In non-statistical contexts express weights and measures in words:
He bought a phial of laudanum and an ounce of arsenic at a pharmacy two miles from Cheapside.
In statistical works or in subjects where frequent reference is made to them, weights and measures may be expressed in figures with appropriate abbreviations:
The priory is situated 3 km from the village of Emshall.
The same 13 mm capitals were used by three Madrid printers at different times.
Note that most such abbreviations do not take a full stop or plural s:
1 kg, 15 kg, 1 mm, 6 cm, 15 m, 4 l (litres), 2 ft, 100 lb, 10 oz
but, to avoid ambiguity, use ‘in.’ for ‘inch(es)’.