Fitzwilliam to Keene

NH 12 begins in Fitzwilliam where it connects to Massachusetts Route 12 at the state line. The highway heads northwest into the town center where it crosses NH 119. NH 12 continues northwest through Troy, then dips into parts of Marlborough and Swanzey before entering the city of Keene. Immediately after crossing into Keene, NH 12 intersects the northern terminus of NH 32 then meets NH 101 near Keene State College. NH 12 turns west along NH 101, then meets NH 10 at a roundabout. NH 10 joins NH 12 and NH 101 west to the next intersection at NH 9. NH 101 ends while NH 10 and NH 12 turn north with NH 9 eastbound in tow. The three routes are multiplexed for 1.2 miles (1.9 km) as they bypass downtown Keene to the west. NH 9 and NH 10 split off at a trumpet interchange while NH 12 continues northwest. The highway interchanges with NH 12A on its way out of town.

Surry to Charlestown

NH 12 enters the southwestern corner of Surry, then turns nearly due west, crossing into the town of Westmoreland. It intersects the northern terminus of NH 63 near the Connecticut River, then turns north and enters Walpole. Now paralleling the eastern bank of the river, NH 12 heads north towards the town center. It intersects with NH 123, which connects to U.S. Route 5 in the neighboring town of Westminster, Vermont. NH 123 south joins NH 12 north and the two routes run in a wrong-way concurrency until crossing the Cold River, where NH 123 splits off to the east. NH 12 continues north along the river and crosses into Charlestown where it meets the northern terminus of NH 12A. The highway continues through downtown, then intersects NH 11, which connects to Interstate 91 and US 5 in nearby Springfield, Vermont. NH 11 east joins NH 12 north and the two routes continue north. A second, northern segment of NH 12A splits off to bypass Claremont while NH 11 and NH 12 head towards the downtown area.

Claremont

In downtown Claremont NH 11 and NH 12 intersect with NH 103 and split after a 10.0-mile (16.1 km) overlap. Facilitated by a traffic circle, NH 11 joins NH 103 eastbound while NH 12 joins NH 103 westbound. NH 12 and NH 103 cross the Sugar River, then parallel it to the northwest (which they do for the remainder of their lengths) back towards the Connecticut River. Heading west, NH 12 and NH 103 cross NH 12A, then turn northwest and cross the river into Ascutney, Vermont. NH 103 ends at the state line while NH 12 becomes Vermont Route 12.

Southern segment

The southern segment of NH 12A is a 21.598-mile-long (34.759 km) loop route of NH 12 which bypasses Westmoreland and Walpole. Its southern terminus is at an interchange with NH 12 in Keene. Its northern terminus is at an intersection with NH 12 in Charlestown.

This section of NH 12A begins at an interchange with NH 12 north of downtown Keene. The highway initially heads northeast, then turns north to parallel the Ashuelot River and enters the town of Surry. NH 12A passes through the center of Surry and continues north into Alstead. In Alstead, NH 12A continues north until intersecting with NH 123. NH 12A joins NH 123 and the highway intersects NH 123A before turning west along the Cold River. The road turns north to cross the river, then NH 12A splits from NH 123. NH 12A continues northwest through the town of Langdon before crossing into Charlestown. NH 12A terminates at NH 12 in the southern end of town.

County	Location	mi	km	Destinations	Notes
Cheshire	Keene	0.0	0.0	NH 12 to NH 9 / NH 10 – Walpole, Charlestown, Keene, Troy	Southern terminus
	Surry			No major junctions
	Alstead	14.5	23.3	NH 123 south (Forest Road) – East Alstead, Marlow	Southern end of concurrency with NH 123
		15.3	24.6	NH 123A east (Acworth Road) – Acworth, Newport	Western terminus of NH 123A
		16.0	25.7	NH 123 north (River Road) – Walpole	Northern end of concurrency with NH 123
Sullivan	Langdon			No major junctions
	Charlestown	21.6	34.8	NH 12 (Bellows Falls Road) – Charlestown, Claremont, Keene	Northern terminus
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Concurrency terminus

Northern segment

The northern segment of NH 12A is a 26.900-mile-long (43.291 km) long spur route of NH 12 that travels south-to-north along the eastern side of the Connecticut River. Its southern terminus is at an intersection with NH 11 and NH 12 in Charlestown. Its northern terminus is at US 4 and NH 10 in Lebanon.

This section of NH 12A starts in northern Charlestown where it splits from NH 11 / NH 12. It runs along the eastern side of the Connecticut River, bypassing downtown Claremont to the west. After crossing the Sugar River, NH 12A crosses NH 12 and NH 103, which cross into Ascutney, Vermont, just to the west.

Now paralleling its parent route, which has become Vermont Route 12 across the river, NH 12A continues north into the town of Cornish. Bridge Street offers a local connection to VT 12, US 5 and VT 44 in neighboring Windsor, Vermont. NH 12A continues through western Plainfield without any major junctions, then enters the city of Lebanon. NH 12A continues north and interchanges with Interstate 89 before terminating at US 4 / NH 10 in West Lebanon village.

County	Location	mi	km	Destinations	Notes
Sullivan	Charlestown	0.0	0.0	NH 11 / NH 12 (Claremont Road) – Claremont, Charlestown, Keene	Southern terminus
	Claremont	7.5	12.1	NH 12 / NH 103 (Main Street) to I-91 / US 5 – Claremont, Newport, Ascutney, VT
	Cornish	12.9	20.8	Bridge Street to I-91 – Windsor, VT
Grafton	Lebanon	25.8	41.5	I-89 to I-91 – Lebanon, Concord, Vermont	Exit 20 on I-89
		26.9	43.3	US 4 / NH 10 (Main Street / Seminary Hill Road)	Northern terminus
1.000 mi = 1.609 km; 1.000 km = 0.621 mi