Anchor Chain Rope Length Calculator
An anchor only holds when the pull on it stays close to horizontal, and that is decided by one number: scope, the ratio of rode paid out to the vertical distance from your bow roller down to the seabed. This anchor chain and rope length calculator does the full marine version of that math. Enter the water depth, the tidal rise you expect, the height of your bow above the water, and your boat length, pick a scope for the conditions, and it returns the total rode to let out, how much of it should be chain, how much rope, and the swinging circle radius you need to keep clear of your neighbours.
That is a different job from a generic chain length calculator, which sizes chain for general use without tides, bow height, scope by conditions, or swing room. If you are anchoring a boat rather than measuring a length of chain, the rode math on this page is the one that keeps you off the beach at 3 a.m.
A calculator cannot feel the bottom. Soft mud, weed, and rock all hold worse than clean sand, and no scope figure fixes a badly set anchor. Back down on it, watch a transit for ten minutes, and add scope whenever the forecast turns.
Quick Answer
Key takeaways
- Scope is measured from the bow roller to the seabed, not from the water surface. Depth, tidal rise, and bow height all count, and leaving the bow height out shortens your rode by the scope times the bow height.
- Conditions pick the ratio: 3:1 for a calm lunch stop, 5:1 for settled weather, 7:1 overnight, 10:1 when it blows.
- The chain portion does two jobs, weight to keep the pull horizontal near the anchor and chafe protection on the bottom. This calculator sizes it at the greater of half the vertical distance or one full boat length.
- Total rode is only half the decision. The swinging circle, rode plus boat length, is the space your boat will sweep as wind and tide turn it.
How much rode do you actually need?
You need enough rode that the anchor is pulled along the bottom, not lifted off it. An anchor digs in and holds when the rode leaves it at a shallow angle. Pay out too little and every wave lifts the shank, works the flukes loose, and starts the slow drag that ends on a lee shore.
The habit that causes most shortfalls is measuring scope from the water surface. Your depth sounder may read 20 ft, but the rode does not start at the waterline, it starts at the bow roller, and the tide does not care what the depth was when you arrived. The BoatUS Foundation anchoring guide defines scope as rode length against the height of the bow above the bottom, and recommends 7:1 for ordinary conditions. This calculator builds the vertical distance the same way: depth, plus the tidal rise expected during your stay, plus bow height, then multiplies by the scope you pick.
The scope formula, step by step
The math runs in four steps, and each step fixes an error people make at the bow with a calculator app in one hand and a rolling deck underfoot.
Build the vertical distance. Add water depth, expected tidal rise, and bow height. Here: 20 + 2 + 4 = 26.0 ft. This is the true height the rode has to span at high water.
Multiply by scope. 26.0 ft at 5:1 gives 130.0 ft of total rode. The ratio comes from the conditions, not from how much line happens to be in the locker.
Split chain and rope. Chain is the greater of half the vertical distance (13.0 ft) or one boat length (35 ft), so 35.0 ft of chain. Rope makes up the rest: 130.0 minus 35.0 = 95.0 ft.
Check the swing. Add the boat length to the rode: 130.0 + 35 = 165.0 ft. That is the radius your boat sweeps around the anchor. If another boat, a shoal, or the shore sits inside it, move or shorten up and accept less scope.
Worked example: a 35-foot boat in 20 feet of water
The calculator above is pre-filled with this case: 20 ft of water, 2 ft of tide to come, bow 4 ft above the water, 35 ft boat, 5:1 scope for a settled afternoon and evening.
| Figure | Value |
|---|---|
| Vertical distance (20 + 2 + 4) | 26.0 ft |
| Total rode at 5:1 | 130.0 ft |
| Chain (greater of 13.0 ft or one boat length) | 35.0 ft |
| Rope (130.0 - 35.0) | 95.0 ft |
| Swinging circle radius (130.0 + 35) | 165.0 ft |
Change only the scope and watch the rode grow: the same anchorage at 7:1 for the night needs 182.0 ft, and at 10:1 for rough weather it needs 260.0 ft, more than many lockers carry. That gap is worth knowing at the dock, not at midnight. It is also why the chain rule uses boat length as its floor: on a bigger boat in shallow water, half the vertical distance would leave only a token few feet of chain, and the anchor would feel every snatch load through stretchy rope alone.
Picking a scope for the conditions
Pick the smallest scope that honestly matches the worst conditions you expect before you next move the boat, not the conditions at arrival. Holding power climbs steeply with scope at first and then levels off, which is why the jump from 3:1 to 5:1 matters far more than the jump from 7:1 to 10:1.
| Scope | Typical use | Rode in 26.0 ft vertical |
|---|---|---|
| 3:1 | Lunch stop in calm water, crew aboard | 78.0 ft |
| 5:1 | Settled weather, good holding ground | 130.0 ft |
| 7:1 | Overnight, unsettled forecast | 182.0 ft |
| 10:1 | Rough weather, poor holding, strong tide race | 260.0 ft |
Bottom type moves the choice as much as wind does. Clean sand and firm mud reward scope generously. Weed lets the anchor skate, and thin sand over rock gives the flukes little to bite, so experienced skippers go a ratio higher and set the anchor harder in both. In a crowded anchorage the honest answer may be a shorter scope plus a second anchor or a different cove entirely, since a textbook 7:1 that swings you into a neighbour is worse than a watched 5:1 that does not.
Chain, rope, or a mix of both?
A mixed rode is the standard setup on cruising boats because each material covers the other's weakness. Chain is heavy, and its weight forms a catenary curve that keeps the pull at the anchor low and horizontal in moderate conditions. It also shrugs off the chafe that rock and coral dish out on the bottom. Rope, usually nylon, is light, cheap per foot, and stretches, which absorbs the snatch loads that would otherwise jerk the anchor free in a chop.
All-chain rodes can run a ratio lower in calm conditions because the weight does more of the work, while rope-only rodes need more scope to achieve the same shallow pull angle, a point the Yacht Yard anchoring guide makes when it prices chain-and-rope setups one step higher than all-chain for the same conditions. Whatever mix you run, size the locker for the deepest anchorage you actually visit: working out the finance side of a boat purchase with a loan for boat calculator is a poor moment to discover the previous owner anchored with 60 ft of rode in 30 ft of water. Owners of larger liveaboard hulls face the same math at bigger numbers, which is why houseboat financing budgets so often undercount ground tackle too.
What competing calculators miss
Winning move 1: the bow height is in the formula. Several rode tools, including the earlier version of this page, multiply scope by water depth alone. Seamanship references are unanimous that scope runs from the bow roller: BetterBoat builds its whole chart on depth plus bow height, and the Accio scope guide writes the formula as depth plus freeboard plus tidal rise, times scope. At 7:1, forgetting a 4 ft bow throws away 28 ft of rode.
Winning move 2: a chain and rope split with a stated rule. The Firgelli tool assumes all chain and stops there. This calculator splits the rode and tells you the rule it used, the greater of half the vertical distance or one boat length, so you can argue with it intelligently instead of guessing a percentage.
Winning move 3: the swinging circle in the same answer. Rode length tells you how to hold. Swing radius tells you whether you may. Most tools leave you to add boat length yourself, usually after you have already dropped the hook in a gap that was too small.
Frequently asked questions
Add your water depth, the tidal rise you expect, and your bow height above the water, then multiply by the scope for your conditions. In 20 ft of water with 2 ft of tide and a 4 ft bow height, the vertical distance is 26.0 ft, so 5:1 scope needs 130.0 ft of rode and 7:1 needs 182.0 ft.
Scope is the ratio of rode paid out to the vertical distance from the bow roller to the seabed. A scope of 7:1 means 7 ft of rode for every 1 ft of vertical distance. Higher scope keeps the pull on the anchor closer to horizontal, which is what makes it hold.
Because the rode starts at the bow roller, not at the water surface. The vertical leg of the triangle runs from the roller down to the anchor. Leaving out a 4 ft bow height at 7:1 scope understates the rode you need by 28 ft, which is a full scope step of holding power lost.
This calculator uses the greater of half the vertical distance or one full boat length, which on the worked example gives 35.0 ft of chain for a 35 ft boat. The chain leader keeps weight at the anchor end and protects against bottom chafe. Boats anchoring over coral or rock often carry more chain than the minimum.
Use 7:1 as the overnight starting point in ordinary conditions, and 10:1 if strong wind is forecast or the holding ground is poor. Overnight means nobody is watching the anchor, the tide will turn the boat, and the weather has hours to change, so the calm-afternoon ratio is not enough.
Your swinging circle radius is your total rode plus your boat length, because the boat can lie in any direction around the anchor as wind and tide shift. The worked example sweeps a 165.0 ft radius. Swinging diameter, the full space you might occupy, is twice that.
Yes, because scope must hold at high water, when the vertical distance is greatest. Anchor at low water in a spot with a 6 ft tidal range and skip the tide in your math, and your 7:1 at arrival quietly becomes roughly 5.6:1 by high water. Enter the expected rise, not just the depth on the sounder.
Often, yes, in calm to moderate conditions. Chain weight creates a catenary that keeps the pull angle low even at shorter ratios, and several cruising guides treat 5:1 all-chain as comparable to 7:1 on a mixed rode. In strong wind the catenary straightens out and the advantage shrinks, so do not bank on it in a blow.
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References
- BoatUS Foundation, "Anchoring and Mooring" study guide - defines scope as rode length against the height of the bow above the bottom and recommends 7:1, with 5:1 acceptable for a lightweight anchor in good weather. BoatUS Foundation: Anchoring and Mooring. Accessed October 2026.
- University of Florida IFAS Extension, Escambia County, "Boating Safety: Anchoring" - states the minimum scope of 5:1, the US Power Squadrons recommendation of 7:1, and worked rode lengths by depth. UF IFAS Extension: Boating Safety, Anchoring. Accessed October 2026.
- WindCheck Magazine, "Weighing the Anchor" - explains scope using depth plus freeboard to the anchor cleat and cites the US Coast Guard recommendation of 7:1 for normal conditions, 5:1 in light air, 10:1 or more in heavy seas. WindCheck: Weighing the Anchor. Accessed October 2026.
- BetterBoat, "Anchor Line Length by Depth" - chart and calculator built on (water depth + bow roller height) x scope, with a warning to correct for what the depth sounder actually displays. BetterBoat: Anchor Line Length by Depth. Accessed October 2026.
- Accio, "Anchor Chain Size Chart" - scope against holding power and pull angle table, and the full formula (depth + freeboard + tidal rise) x scope ratio with a worked example. Accio: Anchor Chain Size Chart and Scope Table. Accessed October 2026.
- Tor Pinney, "Anchoring" cruising guide - the classic statement that depth for scope means depth at high tide plus the height of the hawse or bow roller above the water, with 7:1 for rope-and-chain and 10:1 when a blow is expected. Tor Pinney: Anchoring. Accessed October 2026.