Measuring direct from Blacklunnans to Scurdie Ness near Montrose, and from Cock Cairn to Buddon Ness, which mark the four extreme points of the modern county, we get two lines each 36 miles long. Its area is 559,171 acres or roughly 807 square miles, and it is thus the eleventh in point of size amongst the counties of Scotland. It is less than one-fifth of Inverness-shire, the largest, and more than twenty-two times the size of Clackmannan, the smallest. Its boundary line measures, when zigzags are taken into account, something like 190 miles, of which about 44 are washed by the waters of the ocean and the firth.

Forfarshire is one of the most compact of Scottish counties, being nearly circular in shape. The landward boundary marches with the counties of Perth, Aberdeen, and Kincardine. To the east of the Ericht and the Shee, Perthshire “cuts itself a monstrous cantle out”; Aberdeenshire drives a wedge deeply into its neighbour to the south-east of Loch Muirk ; and in the north-east by its adherence to the inward curve of the North Esk, Forfar loses a crescent-shaped mass of land to Kincardine. With these exceptions the boundary line of the county is fairly symmetrical. The curve of the coast from Dundee to St Cyrus is materially broken only by the promontory of Buddon Ness, the indentation of Lunan Bay, and the tidal waters of Montrose Basin.

The north-west corner-stone of Forfarshire, sometimes given as the huge, rugged mass of Cairn-na-Glasha (3484 ft.), is in reality Glas Maol (3502 ft.), the highest summit of which is in the county. The boundary skirts the edge of the precipices of Creag Leacach as far as Cairn Aigha, then striking direct south it reaches in the parish of Blacklunnans its most westerly point, on the Water of Shee. After following this for a few miles it trends south-east and runs round the Perthshire Hill of Alyth, and in a zigzag course crosses the Isla, beyond which it follows for some distance the Dean. A long tongue of the county then shoots westwards into Perthshire so as to include the parish of Kettins. From this point to the Tay at Invergowrie, the line, which once again projects into Perthshire around the parish of Benvie, is straggling and artificial.

The main range of the Grampians with its lofty but rounded summits divides Forfar on the north from Aberdeen. The principal heights are Tolmount, Cairn Bannoch, Broad Cairn, Dog Hillock, Black Head of Mark, Fasheilach, Mount Keen, Cock Cairn, Hill of Cat, and Mudlee Bracks. On Mount Battock the two counties are met by a third—Kincardine. Thence, passing Sturdy Hill, the county march descends to the North Esk at The Burn, and follows the course of that river to the sea.

Southward to Buddon Ness, the county is bounded by the North Sea ; and then westward to Invergowrie, by the estuary of the Tay.