Updated November 2026 · free, works in your browser
Show the formula
Height = water depth + tide allowance + bow height
Rode = height × scope ratio
Distance to anchor ≈ √(rode² − height²), a straight-line estimate that ignores sag and rode lying on the seabed.
To work out how much anchor rode to let out, add the water depth, the expected tide rise and the height of your bow above the water, then multiply by the scope ratio. In a calm anchorage a ratio of 5:1 is a common starting point, so 8.5 m of total height means about 42.5 m of rode. The calculator does this instantly in metres or feet.
How to use the anchor scope calculator
- Choose metres or feet. If you switch units, the numbers you already entered are converted for you.
- Enter the water depth at the spot where you plan to anchor, measured from the surface to the seabed.
- Enter the bow height: the distance from the water to the bow roller or fairlead where the rode leaves the boat.
- Enter a tide allowance: how much the water is expected to rise between now and the highest tide while you are anchored. Use 0 where there is no meaningful tide.
- Pick a scope ratio. The result shows the rode to let out, plus the height used, the estimated distance from the anchor to the bow, and the length in the other unit.
How it is calculated
Scope is the ratio between the length of rode you pay out and the vertical height from the bow roller to the seabed. This is standard seamanship practice, and the formula is simple:
Height = water depth + tide allowance + bow height
Rode = height × scope ratio
The distance from the anchor to the bow is estimated with Pythagoras: the square root of (rode² − height²). It assumes a straight rode, so in reality it is slightly shorter when the rode sags or lies along the bottom. Metres and feet are converted with 1 m = 3.28084 ft, and results are rounded to one decimal place.
Typical scope ratios
| Ratio | Typical use |
|---|---|
| 3:1 | Short stop in calm, sheltered water with someone aboard |
| 5:1 | Common everyday ratio for settled weather |
| 7:1 | Overnight stays, fresher wind or some swell |
| 10:1 | Strong wind or poor holding, where space allows |
These are general rules of thumb, not guarantees. Seabed type, anchor design, rode material and the forecast all matter, and an all-chain rode usually holds better than rope at the same ratio. Always follow your anchor maker's advice and use your own judgement.
Frequently asked questions
How much anchor chain do I need?
It depends on the deepest water you expect to anchor in. Multiply the total height by the ratio you want, then carry a rode at least that long. For example, 10 m of depth, 1 m of tide and 1.5 m of bow height at 7:1 comes to 87.5 m.
Do I measure depth from the surface or from the bow?
Scope is measured from the bow roller, not the surface. That is why the bow height is added to the depth. Skipping it gives a rode that is too short, especially on boats with a high bow.
Should I allow for the tide?
Yes. Rising water increases the height your rode must cover, so allow for the rise between dropping the anchor and high tide. If you anchor near high water, you may use little or no allowance.
Is a bigger scope always better?
A longer rode gives a lower pull angle at the anchor and usually better holding. It also makes your boat swing in a larger circle, which may not fit in a busy bay. Balance holding against the space around you.
Why is the distance to the anchor shorter than the rode?
The rode runs diagonally from the bow down to the seabed, so the horizontal distance is shorter than its length. The estimate is a straight-line figure and ignores sag.
Anchoring is part of the fun on a sailing holiday, and knowing your rode length lets you enjoy a quiet bay with confidence. Once the water is sorted, it is worth planning your time ashore too: many charter guests pair a night at anchor with a stay in a nearby coastal town before or after the trip, so a comfortable hotel near the marina makes a good bookend to the voyage.
Other tools
- Passage Time CalculatorEnter distance and average speed to get passage duration and arrival time for any boat leg.
- Beaufort Scale to Knots ConverterTurn a Beaufort wind force into knots, km/h, mph and sea state, or find the force for any wind speed.
- Sail Performance CalculatorCalculate displacement/length ratio, sail area/displacement ratio, capsize screening value and hull speed from a boat's dimensions.