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Scope {{ resultsReady ? `${computation.values.selected_scope}:1` : '—' }} Rode carried {{ resultsReady ? (computation.values.adequate ? 'Meets selected scope' : 'Shortfall') : '—' }}
Use surface-to-bottom depth. Correct a sounder reading for its transducer offset.
Additional rise above the entered depth during your stay; use 0 if depth already includes high water.
Waterline to the roller or cleat where the rode leaves the boat.
Overall length, used to extend the swing estimate from bow to stern.
Usable chain plus rope for this anchor, excluding any length you cannot pay out.
Choose guidance appropriate to your vessel and conditions. These ratios do not establish that an anchorage is safe.
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Rode length divided by bow-to-bottom height. Lower ratios are calculations, not recommendations.
:1
Optional additional distance beyond the rode-and-boat bound; 0 adds no buffer.

Enter extra clearance from 0 to 1,000 m (3,280.84 ft). Required rode remains available; swing clearance is unavailable until corrected.

Optional actual-scope check at the expected highest water level. Use 0 to leave this check off.

Enter a deployed length from 0 to 20,000 m to restore the optional scope check.

Anchoring estimate
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Derived lengths and the scope chart follow the water-depth unit. The ledger keeps each input measurement in its chosen unit.

Correct extra clearance in Advanced to restore the swing estimate.

Swing planning uses total rode + boat length + your extra clearance. It allows more room than the idealized high-water horizontal reach; it does not allow for stretch, anchor drag or moving neighbors.

{{ scopeGuidance }} Check holding ground, forecast wind and water levels, anchor and rode specifications, and room throughout the swing. Set and monitor the anchor; no ratio guarantees holding.

Scope guidance: BoatUS Foundation. Tide example: USCG Auxiliary. These are seamanship references, not a regulatory minimum.

Calculation method
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The chart is unavailable. The anchorage ledger includes all comparison values.

Measurement Value Unit / basis Copy
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A rising tide can reduce an anchored boat's scope without any rode being hauled in. The bow moves farther above the bottom while the paid-out length remains unchanged. An anchoring plan therefore needs the expected highest water level during the stay, not just the depth on arrival.

Rode
The chain, rope, or combination connecting the boat to its anchor. A rode-length calculation counts the chain and rope together.
Bow-to-bottom height
The vertical distance from the seabed to the point where the rode leaves the boat. It includes water depth and the attachment height above the waterline.
Scope
The ratio of paid-out rode length to bow-to-bottom height. A 7:1 scope means seven units of rode for each unit of that vertical height.

Bow height matters because the rode does not end at the water surface. Applying a scope ratio to water depth alone understates the length needed for that ratio. At 7:1, every additional metre of bow-to-bottom height requires another seven metres of rode, whether that height comes from deeper water, a rising tide, or a higher bow attachment.

The amount carried and the amount deployed answer different questions. Usable rode carried determines whether a planned payout is possible. Only the length actually paid out determines the deployed scope. Spare chain or rope still aboard does not improve that ratio, and length that cannot be paid out should not be counted as available.

Clearance needs a separate check. With the same rode paid out, falling water can increase the horizontal distance between the anchor and the bow even though scope increases. The stern extends beyond the bow, and a change in wind or current can swing the boat toward an obstruction that was previously well clear. High-water scope and room throughout the swing are therefore different parts of the anchoring decision.

More scope allows a shallower idealized pull, but paying out more rode also uses more space. The appropriate choice depends on the vessel, anchor and rode specifications, holding ground, and expected conditions. A scope ratio does not establish anchor holding power or certify a safe anchorage. The anchor still needs to be set and monitored.

How to Use This Tool:

Use measurements for the anchorage and the period you intend to remain there. Each length can use metres or feet independently, and results update as you edit.

  1. Enter Water depth as surface-to-bottom depth and Expected tide rise as the additional rise above that depth during your stay. Correct a sounder reading for its reference offset. Use 0 for the rise when the entered depth already covers the expected highest water level.
    With charted depth, use the highest predicted water level above the same chart datum as the tide allowance. Do not substitute only the rise remaining from the present time or add the same allowance twice.
  2. Enter Bow attachment height, overall Boat length, and Rode available. Measure bow height from the waterline to the roller or cleat where the rode leaves the boat. Available rode means the total usable chain and rope for this anchor, not just the amount remaining aboard after some has been deployed.
  3. Choose Scope guidance. The initial selection is General guidance — 7:1. For an exact alternative, choose Custom scope and enter a ratio from 1:1 to 15:1.
  4. Open Advanced to add Extra swing clearance or check Rode already deployed. Extra clearance adds distance to the swing radius, not to the required rode. A positive deployed length enables a scope check at the expected highest water level. Leave either value at 0 when it is not needed.
    Invalid extra clearance hides the swing estimates until corrected. An invalid deployed length disables only its optional scope check. Neither error removes the required-rode result when the basic measurements are valid.
  5. Read Rode for selected scope and the Rode carried status. If Check the anchorage inputs appears, correct the field named in the message. Before relying on the length, confirm that the displayed bow-to-bottom height represents the expected highest water level.

Interpreting Results:

Rode for selected scope is the total chain-and-rope length required by the chosen ratio. It is not an additional length to add to your chain. Meets selected scope means the usable carried length covers that requirement. Shortfall means it does not, and the accompanying message gives the missing length and the scope available if all usable rode were deployed.

The guidance choices select ratios, not a calculation of weather severity or anchor performance. Their qualifications matter when deciding which comparison is appropriate.

Guidance choice Scope Meaning and limit
General guidance 7:1 A customary general reference, not a vessel-specific holding assessment.
Small boat, good weather 5:1 A conditional example for a small boat with a lightweight anchor in good weather. It is not a universal all-chain rule.
More scope reference 10:1 An additional-scope comparison within the referenced guidance range, not a storm-safety prescription.
Custom scope 1:1 to 15:1 Your selected ratio. Accepting the number does not endorse it for the conditions.

These references are not regulatory minimums. A satisfactory carried-length result still needs to be checked against the actual holding ground, forecast conditions, equipment specifications, and available room.

Idealized high-water reach measures horizontal anchor-to-bow distance under a straight, taut-rode assumption. Conservative swing radius allows for the required rode, the boat's overall length, and your extra clearance. Its centre is the anchor, and Swing planning diameter is twice that radius. Neither number identifies hazards or measures the separation from another boat.

The swing estimates use the required rode for the selected scope, not all rode carried and not the optional deployed length. A shortfall does not shrink the displayed plan to match your inventory. Likewise, entering a longer deployed length does not enlarge the swing estimate. Recheck clearance for the length actually paid out. The deployed entry is also checked independently of the available entry, so reconcile those two measurements yourself.

The comparison includes 5:1, 7:1 and 10:1 requirements, plus a distinct custom ratio. Read the numerical carried-length check even when no carried reference line appears on the chart. Length results follow the water-depth unit; input measurements retain their own units in the ledger.

Technical Details:

Required rode grows directly with both bow-to-bottom height and the selected scope ratio. Chain and rope contribute to one total length. No material-specific factor, load calculation, or automatic weather allowance changes that multiplication.

Rode length at high water

First add water depth, expected rise, and bow attachment height. Then multiply the combined height by the selected scope.

H = d+t+b L = S×H

Here, d is Water depth, t is Expected tide rise, and b is Bow attachment height. Their sum, H, is the bow-to-bottom height at the expected highest water level. S is the dimensionless selected scope, and L is Required rode. All lengths must share one unit before addition.

Both tide rise and bow height are included before multiplication. Boat length and extra swing clearance do not change the required rode. Increasing the available or deployed length also leaves that requirement unchanged.

Carried and deployed scope

Let A be usable Rode available and P be a valid positive Rode already deployed. Dividing either by the same high-water height gives its scope. Subtracting the required rode from the available rode gives the carried-length margin.

Scarried = AH Sdeployed = PH M = A−L

A positive margin is spare usable length, and a negative margin is a shortfall. Equality meets the selected ratio. Comparisons use unrounded lengths with a tolerance of 0.000000001 m, or 10−9 m, solely to accommodate numerical roundoff. This is not an operational safety allowance.

Check Condition before display rounding Outcome
Carried length M ≥ −10−9 m Meets selected scope
Carried length M < −10−9 m Shortfall
Deployed length Valid P > 0 and P − L ≥ −10−9 m The deployed ratio meets the selected ratio.
Deployed length Valid P > 0 and P − L < −10−9 m The deployed ratio is below the selected ratio.
Deployed length P is 0 or invalid. No actual-scope comparison is performed.

Horizontal reach and swing geometry

A straight, taut rode forms the sloping side of a right triangle. Its vertical side is the bow-to-bottom height, so the remaining side gives the idealized horizontal reach x. A separate length bound uses the full required rode, overall boat length B, and extra clearance C to obtain swing radius R and diameter D.

A taut rode forms a right triangle with vertical bow-to-bottom height and horizontal anchor-to-bow reach.
Straight, taut rode geometry; not to scale.
x = max ⁡ ( 0 , L2 − H2 ) R = L+B+C D = 2×R

The maximum with zero keeps the square root nonnegative. Accepted scope ratios already make L at least as large as H. At the mathematical limit of 1:1, the idealized horizontal reach is zero; that boundary is not an anchoring recommendation.

Using the full rode length rather than the shorter high-water horizontal reach leaves more room for the boat, including when water falls. The bound assumes a fixed anchor and rode that does not stretch. Adding the boat's overall length allows for the vessel beyond the bow, but does not model its precise orientation. Anchor drag, rode stretch, and moving neighbouring boats can invalidate a clearance assessment based only on this geometry. No amount of extra clearance is selected automatically.

Accepted measurements and units

The following limits are inclusive. They define accepted entries, not suitable operating conditions or a validated vessel-size range. Basic measurements require finite numeric values, including an explicit 0 where zero is allowed.

Measurement Minimum Maximum Unit
Water depth 0.1 1,000 m
Expected tide rise 0 100 m
Bow attachment height 0 30 m
Boat length 0.1 300 m
Rode available 0 20,000 m
Extra swing clearance 0 1,000 m
Rode already deployed 0 20,000 m
Custom scope 1 15 Ratio :1

The custom-scope limit applies only when selecting that ratio. It does not cap the scope calculated from available or deployed rode.

Feet are converted to metres before the calculations, using exactly 0.3048 metres per foot. For the same physical length, lm is its numerical value in metres and lft is its numerical value in feet.

lm = 0.3048 × lft

Calculations retain unrounded 64-bit floating-point values. Measurement fields display up to 12 significant digits, and changing units preserves the underlying physical length. Length results and calculated carried or deployed ratios display up to two decimal places. Comparisons take place before that rounding, so displayed equality can conceal a small shortfall. There is no automatic upward rounding to a chain mark or other practical payout increment.

Allowing for a rise during the stay

With 5 m of water depth, an expected 2 m rise, and a bow attachment 1 m above the waterline, bow-to-bottom height is 8 m. Selecting 7:1 requires 56 m of total rode. Carrying 60 m leaves a 4 m length margin and would provide 7.5:1 if all usable rode were deployed. For a 10 m boat with no extra clearance, the conservative swing radius is 66 m and the diameter is 132 m. The idealized high-water reach is only about 55.43 m because it measures anchor-to-bow distance, not the room allowed for the entire boat. The 4 m remaining aboard is spare length, not proof of holding or clearance.

H=5+2+1=8 m L=7×8=56 m

Limitations and Privacy:

The estimate does not calculate anchor holding capacity, rode strength, or wind, wave, and current loads. It does not select an anchor, size chain or rope, or evaluate whether the seabed is suitable. Boat length affects the swing allowance only; it is not used to assess equipment capacity.

Tide rise and water depth are supplied by you. The calculation does not retrieve tide predictions, weather forecasts, or charts. It also does not check the boat's draught or depth beneath the hull at low water, so a satisfactory high-water scope result cannot rule out grounding later in the stay.

The arithmetic runs in your browser. Measurements and scope settings can be included in the page address and restored on reload, so sharing or saving that address may retain them. The comparison chart loads an external resource when needed. Browser-based calculation should not be read as a guarantee that the whole page makes no network requests or that address-carried inputs remain private.

References: