Boat Oars: Length Sizing and Oarlock Fit

Boat Suppliers

18 October 2026

Boat oars are sized off the boat, not off the rower: measure the span between the oarlock sockets, take half of it and add 2 in, and that is the inboard length from the grip to the oarlock. Multiply the inboard by 25 and divide by 7 for the overall oar length. A 48 in (122 cm) span, typical for a 9 to 10 ft dinghy, comes out to 93 in, so you buy 7-1/2 or 8 ft oars. That formula, plus the shank diameter of your oarlock sockets, settles almost every oar purchase. Here is the rest of it.

Boat oars length by oarlock span

The 25:7 ratio is the old Shaw and Tenney rule, and it holds up because it is really a gearing rule: 7 parts inboard, 18 parts outboard, so the blade moves about 2.5 times as far as your hands. Rounding up gives a longer outboard, more distance per stroke and a heavier pull, which is what you want in a loaded skiff with a slow cadence. Rounding down gives a lighter, quicker stroke that a smaller rower can hold for an hour. Between two standard sizes, take the longer oar for a heavy or beamy boat and the shorter one for a light tender you row against a chop.

Span between oarlocks	Inboard (half span + 2 in)	Calculated length	Buy
36 in (91 cm)	20 in	71 in	6 ft
42 in (107 cm)	23 in	82 in	7 ft
48 in (122 cm)	26 in	93 in	7-1/2 or 8 ft
54 in (137 cm)	29 in	104 in	8-1/2 ft
60 in (152 cm)	32 in	114 in	9-1/2 ft
66 in (168 cm)	35 in	125 in	10 or 10-1/2 ft

Measure span socket center to socket center, not gunwale to gunwale; on a boat with outrigger-style oarlock pads the difference is 4 to 6 in and it changes the answer by a full foot. Drift boats and rafts run their own charts by boat width and rower position; use the builder's chart for those.

Oarlock fit: shank diameter and horn opening

An oar that is the right length and does not fit the oarlock is still the wrong oar. Two numbers matter. The shank of the oarlock has to fit the socket in the boat: 1/2 in (12.7 mm) is the common size on small boats, some older boats and imported dinghies have 7/16 in sockets, and heavier skiffs run 5/8 in. Measure the socket bore with calipers. A 1/2 in shank rattling in a 5/8 in socket wears both out in a season; the fix is a new socket or a bushing, not tape.

The horn opening has to clear the oar shaft at the collar, which on a 7 to 8 ft oar is about 1-5/8 to 1-3/4 in (41 to 44 mm) in diameter, with a leather or a rubber collar over it. A standard round horn oarlock takes up to about a 2 in shaft; ribbed horns grip a collar with a matching button and stop the oar sliding outboard, and a clamp-on or pinned oarlock captures the shaft entirely. Captured oars cannot be feathered and cannot be lost overboard. For a tender that gets rowed by whoever is on the boat, captured is the right call. For a boat you actually row for distance, open horns and leathers.

Oarlock type	Feathers	Can lose the oar	Best for
Round horn	Yes	Yes	Rowing for distance, traditional boats
Ribbed horn with button	Yes	Rarely	Dinghies, general use
Clamp-on or pinned	No	No	Tenders, kids, rental boats

Wood, aluminum, or composite

Spruce oars are the light ones, around 3 to 4 lb (1.4 to 1.8 kg) for a 7 ft oar, and they flex, which saves your shoulders on a long pull; they also ding easily and need finish maintenance. Ash is tougher and about a pound heavier per oar, the choice for a workboat that gets used to fend off pilings. Aluminum shaft oars with plastic blades are cheap, stiff, cold in the hand, and need nothing but a rinse; they are the right oars for a tender that lives upside down on the foredeck. Carbon and fiberglass composite oars are the lightest and the most expensive, and only make sense if you row for exercise.

Collars and buttons: setting the inboard

The collar, leather or rubber, protects the shaft where it rides in the oarlock, and the button on the collar stops the oar sliding out. Set it from the table: the button sits just outboard of the oarlock when the grip end is at the inboard length, which for a 48 in span is 26 in from the grip end to the oarlock centerline. Leathers run 10 to 12 in long so you have 4 to 6 in of adjustment. The 2 in you added to half the span builds in a 4 in overlap of the grips at the crossover, which is correct; you row one hand above the other. If the overlap is more than a hand's width, move the buttons out an inch at a time.

Refinishing wooden oars

Wooden oars fail at the finish, not the wood. Once the varnish goes, water gets into the end grain at the blade tip, the tip splits and the blade checks. The maintenance schedule is one fresh coat of spar varnish at the start of each season, and a full refinish when the varnish has gone cloudy or lifted, roughly every 3 to 5 years.

The refinish: sand back to sound finish or bare wood at 80 grit, then 120, then 150 with the grain; do not go finer or the first coat will not bite. Fill dings and checks with epoxy thickened with fumed silica, pressed in with a 1-1/4" Stainless Steel Putty Knife, and seal the blade tips with two coats of unthickened epoxy so the end grain stops drinking. Then spar varnish, minimum four coats, sanded at 220 between coats. A 2" Foam Brush - 48 Pack handles the shaft and a 3" Foam Brush - 36 Pack lays the blades flat and fast with no brush marks and no shed bristles, and you throw each one away instead of cleaning it. Leave the grips bare; varnished grips raise blisters in the first mile, so rub them with a little oil or leave the wood alone. Sanding old varnish and epoxy wants a P100 respirator, nitrile gloves, and eye protection. Store oars horizontal, out of the sun, and never blade-down in the bilge.

If you are rebuilding the boat along with the oars, check the flotation under the thwarts while the seats are out; a waterlogged chamber gets cut open, dried, and refilled with 2 lb closed-cell pour foam, mixed 1:1 by volume and poured in lifts, about 1 cu ft per quart kit.

Frequently asked questions

How long should oars be for a 10 foot boat?

It depends on the beam at the oarlocks, not the length. Most 10 ft dinghies span 46 to 50 in between oarlock sockets, which puts them on 7-1/2 to 8 ft oars: half the span plus 2 in is the inboard, times 25 divided by 7 is the length. A narrow 10 ft pram at 42 in span drops to 7 ft oars. Measure before you buy.

What size oarlocks do I need?

Measure the socket bore, not the oar. Most small-boat sockets are 1/2 in; older and imported dinghies sometimes use 7/16 in, and heavy skiffs use 5/8 in. Then match the horn opening to your oar shaft at the collar, usually 1-5/8 to 1-3/4 in on a 7 to 8 ft oar. If you can, buy the oarlocks and sockets as a set so the shank fit is right from the start.

Are wooden oars or aluminum oars better?

For a tender that gets tossed on deck and rowed 200 yards to the dock, aluminum with plastic blades: no maintenance, no rot. For rowing any real distance, spruce or ash. A 7 ft spruce oar weighs about 3 to 4 lb and flexes through the stroke, which aluminum does not. Wood needs a coat of varnish every season; that is the whole cost.

How many coats of varnish do oars need?

Four coats of spar varnish minimum on a full refinish, sanded at 220 between coats, over blade tips sealed with two coats of epoxy. Then one maintenance coat every season before the varnish dulls. Use a foam brush and throw it away after each coat. Keep the grips bare; varnished grips blister your hands within a mile.

Should oars be pinned or free?

Pinned or clamp-on oarlocks for a tender, a kids' boat, or anything rowed by a guest, because the oar cannot go overboard and it always sits at the right inboard. Free oars in open horns for anyone who rows for distance, because you can feather the blade on the return and ship the oars the moment a wake comes. Most tenders under 10 ft should be pinned.

Where should the oar collar go?

Set the button so it rides just outboard of the oarlock when the grip end is at the inboard length: half the oarlock span plus 2 in. On a 48 in span that is 26 in from the grip end. That gives a 4 in overlap of the grips at the crossover, which is correct. Leathers 10 to 12 in long leave room to adjust.