Oar Length Calculator
An oar that is too long turns every stroke into a grind; one that is too short slips through the water and wastes your effort. The right length follows from two measurements most rowers already have: the width of the boat and the distance between the oarlocks. This calculator runs both respected sizing methods, beam-based and oarlock-span-based, including the inverse check for oars you already own.
Quick Answer
For a racing shell with a 5-foot beam rowed by someone 70 inches tall, the calculator recommends 9.50 ft (114.0 in), with no height adjustment. With the span method, a 63-inch oarlock span gives 119.64 inches, just under 10 feet (303.9 cm). The calculator above is pre-filled with the beam-method example.
Key takeaways
- Two respected methods, one page: size from the boat’s beam by boat type, or from the oarlock span with the classic Shaw and Tenney rule.
- The span method also runs in reverse: enter oars you already own to find the rigging width they suit.
- Beam is measured at the oarlocks, oarlock center to oarlock center, not the widest point of the hull.
- Always round the recommendation to the nearest oar size actually sold, then check it against your rigging before buying.
How to use the oar length calculator
The beam method answers from the boat’s width and type; the span method answers from a direct measurement between the oarlocks. Pick the tab that matches the measurement you have. Results stay hidden until you click Calculate Oar Length.
- Beam method: enter the boat’s beam in feet, your height in inches, and the boat type closest to yours. The pre-filled run is a racing shell with a 5-foot beam and a 70-inch rower, which returns 9.50 ft (114.0 in) with no height adjustment.
- Height adjustment: the tool trims 0.3 ft for rowers under 66 inches and adds 0.3 ft for rowers over 72 inches. A 64-inch rower on the same shell gets 9.20 ft; a 74-inch rower gets 9.80 ft.
- Span method: measure the oarlocks center to center and enter the number in inches or centimeters. A 63-inch span returns 119.64 inches (9.97 ft, 303.9 cm).
- Inverse check: switch Direction to “Oar length → matching span” and enter oars you already own. Ten-foot (120-inch) oars suit a 63.20-inch span.
- Round and rig: oars are sold in fixed sizes, so round the recommendation to the nearest available length and confirm it against your boat’s rigging before you buy. If you are still shopping for the hull itself, the boat price calculator breaks down what the sticker price really covers.
The oar length formulas
The beam method scales a per-boat-type formula with the boat’s width and nudges it for the rower’s height. The span method is the classic Shaw and Tenney rule, used by oar makers since 1858, and the inverse is its exact algebra run backward.
Height adjustment: under 66 in − 0.3 ft · over 72 in + 0.3 ft · 66–72 in no change
Span method: Oar length = (Span ÷ 2 + 2) × 25 ÷ 7 (span in inches)
Inverse: Span = (14 ÷ 25) × Oar − 4
Metric: centimeters = inches × 2.54
| Boat type | Formula | At a 5-ft beam |
|---|---|---|
| Racing shell | (Beam × 1.5) + 2 ft | 9.50 ft |
| Recreational rowing | (Beam × 1.4) + 1.5 ft | 8.50 ft |
| Sailing dinghy | (Beam × 1.3) + 1 ft | 7.50 ft |
| Kayak paddle | (Beam × 1.2) + 0.5 ft | 6.50 ft |
| Canoe paddle | (Beam × 1.25) + 0.75 ft | 7.00 ft |
Read the right-hand column as a bar chart and the pattern is obvious: narrower, faster boats take longer oars, while the kayak and canoe rows are paddles, a different sizing problem the beam formula only approximates.
Worked example: three verified runs
Every figure below is the calculator’s exact output, checked against the code. The first is the pre-filled run; the other two show the span method forward and in reverse.
| Run | Inputs | Result |
|---|---|---|
| Beam method | 5-ft beam, racing shell, 70-in rower | 9.50 ft (114.0 in), no height adjustment |
| Beam, shorter rower | 5-ft beam, racing shell, 64-in rower | 9.20 ft (110.4 in), −0.3 ft |
| Span method | 63-in oarlock span | 119.64 in (9.97 ft, 303.9 cm) |
| Inverse check | 120-in oars you already own | Matching span 63.20 in (5.27 ft, 160.5 cm) |
Inboard, outboard, and the handle gap
Every oar pivots on the oarlock, and where that pivot sits decides how the oar feels. Inboard is the handle-to-oarlock part; outboard is oarlock to blade tip. The Shaw and Tenney rule builds in a 7:18 split, about 28% inboard, which their sizing guide calls the ideal ratio for almost all boats.
That ratio is why the formula’s constant looks the way it does: (span/2 + 2) is the inboard loom length in inches, and multiplying by 25/7 stretches it to the full oar. Get the inboard wrong and the oar tells you immediately: too little inboard and the blade feels heavy, too much and you lose leverage. Concept2’s own guidance makes the same point from the rigging side, noting that a longer oar raises the gearing, so faster crews and efficient blades call for shorter settings. If you prefer an overlapping grip instead of the usual 1 to 3 inch gap between the hands, Shaw and Tenney’s advice is to add 6 inches to the calculated length. When a hull is only an idea so far, the boat value calculator gives you a reality check on secondhand prices, and the marine loan calculator prices the financing if you buy new.
What competing calculators miss
Every ranking calculator commits to exactly one method, and most of them hide the working. Here is what the alternatives do and where they stop:
- Calculator Ultra’s oar length calculator: span-only, using the same (S/2 + 2) × 25/7 formula with a 50-inch worked example. No beam method, no inverse span check, no metric conversion, no height adjustment.
- BraveCalculator’s oar length calculator: the beam side only, built on the inboard-ratio formula (oar = 2 × inboard + beam at the oarlocks) with a rowing-style selector and a 60-inch beam example. No span method, no inverse check.
- Colorado Kayak’s raft sizing guide: the practical 3/2 rule for raft frames (half the frame width times three) with sensible advice on when to round up or down. Raft-specific; it does not cover rowing shells or oarlock spans.
- The Swampscott Dory oar page: honest fixed-seat dory numbers with the Shaw and Tenney attribution, but it is a blog page, not a tool.
- Forum rule collections: threads of traditional rules, half the beam times three plus 6 inches and its cousins, which contradict each other and cannot be checked against your own measurements.
- The gap this page fills: both methods on one page, the inverse span check (the exact algebra of the Shaw and Tenney rule, not a guess), metric in and out, a stated rower-height adjustment with its 66 and 72 inch boundaries, and copy and share on every result.
Frequently asked questions
The widely cited rule is the Shaw and Tenney formula, used by oar makers since 1858: oar length = (oarlock span ÷ 2 + 2) × 25 ÷ 7, with the span measured in inches. A 63-inch span gives about 119.6 inches, or just under 10 feet.
Measure straight across the boat at the oarlocks, from one oarlock center to the other. That is the number the formulas want, not the widest point of the hull if the oarlocks sit elsewhere.
Inboard is the part of the oar from the handle to the oarlock; outboard is from the oarlock to the blade tip. The Shaw and Tenney rule builds in about 28% inboard (a 7:18 split). Too little inboard and the oar feels heavy; too much and you lose leverage.
Most rowers use a small gap of 1 to 3 inches between the hands on the return stroke. An overlapping grip is also used and feels efficient to some rowers; Shaw and Tenney’s guidance is to add 6 inches to the calculated length if you row with an overlap.
Use the beam method when you know the boat’s width and type but cannot easily measure between the oarlocks, for example when shopping for a boat. Use the span method when the oarlocks are in front of you and you can measure center to center. For the same boat the two answers should land within a few inches of each other.
The calculator takes 0.3 ft off for rowers under 66 inches and adds 0.3 ft for rowers over 72 inches, with no adjustment between 66 and 72 inches. The adjustment is small on purpose: height fine-tunes the length, while the boat’s beam decides it.
Not really. Kayak paddles are sized mainly by the paddler’s height and torso length plus the kayak’s width, typically 210 to 230 cm for touring. The beam formula on this page gives only a rough starting point for paddles, which the kayak option’s tooltip says plainly.
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References
- Shaw & Tenney. How to Properly Size Rowing Oars. The original span formula in use since 1858, the 7:18 inboard-to-outboard ratio, the 1 to 3 inch hand gap, and the 6-inch allowance for an overlapping grip. Accessed October 23, 2026.
- Concept2. Checking and Setting Oar Length. How overall oar length changes gearing and loading, and when to set oars shorter. Accessed October 23, 2026.
- Concept2 Help Center. How Do I Check or Measure Oar Length?. Measuring from the end of the grip to the end of the blade, and why a longer oar gives a higher gear. Accessed October 23, 2026.
- Colorado Kayak Supply. How to Choose the Right Oar Length for Your Raft. The 3/2 rule for raft frames and when to round the calculated length up or down. Accessed October 23, 2026.
- Glen-L Marine. About Oar Length. Traditional rules of thumb, including the twice-the-beam-plus-6-inches rule and the half-beam divided by 7 multiplied by 25 variant. Accessed October 23, 2026.
- Ready All Row. Oars: Oar Length and Inboard. Oar length measured grip to blade tip and how inboard affects how heavy the boat feels. Accessed October 23, 2026.