Draught Survey Cargo Calculator
Use this quick calculator to estimate cargo loaded or discharged between initial and final draught surveys. Enter hydrostatic displacement values and onboard non-cargo weights in metric tonnes.
Assumption: hydrostatic table displacements are referenced to seawater density 1.025 t/m³.
What is a draught survey calculation?
A draught survey calculation is a marine method used to estimate cargo quantity by comparing a vessel’s displacement at two points in time: typically before loading/discharging and after loading/discharging. It is one of the most widely used cross-check methods in bulk shipping, especially when shore scales are unavailable or when parties need an independent quantity reference.
In simple terms, if a ship sits deeper in the water after operations, its displacement has increased. After correcting that displacement change for water density and non-cargo weight changes (fuel, ballast, fresh water, stores), the remaining weight change is treated as cargo movement.
Core formula used in this calculator
Corrected displacement = Observed displacement × (Observed water density / 1.025)
Cargo change = (Final corrected displacement − Initial corrected displacement) − (Final non-cargo − Initial non-cargo)
If cargo change is positive, it indicates cargo loaded. If negative, it indicates cargo discharged.
Data required for reliable draught survey work
1) Accurate draught readings
Take forward, midship, and aft draughts on both port and starboard where possible. Correct readings for waves, swell, and meniscus effects. Readings should be taken with enough repetition to reduce random error.
2) Water density measurements
Measure dock water density at multiple locations and depths near the vessel. Density can vary significantly due to tide, fresh water inflow, and terminal conditions. Small density errors can create large quantity differences in deep-draft ships.
3) Hydrostatic reference data
Use the vessel’s approved hydrostatic tables (or loading computer output) for displacement, TPC, MTC, LCF, and trim/list corrections. Ensure the same basis is used for both initial and final surveys.
4) Soundings and tank data
Record all relevant non-cargo liquids and constants:
- Fuel oil / diesel / lube oil
- Fresh water
- Ballast water
- Slops and bilge where applicable
- Stores and consumables if materially changed
Step-by-step draught survey workflow
- Take initial draught readings and water density.
- Derive initial displacement from hydrostatic data and apply corrections (trim/list/hog-sag if required by your procedure).
- Conduct full tank soundings and compute initial non-cargo weights.
- Repeat all readings at final condition after cargo operations.
- Apply density correction to both displacements.
- Calculate displacement change and deduct non-cargo change.
- Reconcile with terminal figures, bill of lading, and operational logs.
Worked example (simplified)
| Item | Initial | Final |
|---|---|---|
| Observed displacement (t) | 22,345.50 | 38,790.20 |
| Water density (t/m³) | 1.012 | 1.018 |
| Total non-cargo on board (t) | 5,410.00 | 2,020.00 |
After density correction, displacement change is computed, then corrected for non-cargo reduction. The resulting value gives estimated cargo loaded. This is exactly the logic implemented in the calculator above.
Common errors in draught survey calculations
- Density mismatch: using a single default value instead of measured dock water density at both times.
- Poor draft mark visibility: incorrect mark reading due to swell, paint condition, or parallax.
- Unreconciled ballast records: tank transfers not logged correctly during operations.
- Ignoring trim/list effects: using uncorrected mean draft where vessel attitude is significant.
- Unit mistakes: mixing kg/m³, t/m³, and metric tonnes inconsistently.
Best practices for better accuracy
Use repeat measurements
Take multiple density samples and repeated draft observations. Average values reduce random noise.
Synchronize timing
Align draft readings, tank soundings, and density sampling as closely as possible in time to avoid drift caused by consumption or transfer operations.
Document assumptions
Keep a clear calculation sheet noting correction methods, hydrostatic source, density values, and all included/excluded tank weights. This helps during charter-party quantity discussions.
How to interpret calculator output
The calculator returns:
- Initial and final corrected displacement
- Displacement change between surveys
- Total non-cargo changes
- Estimated cargo loaded/discharged
Use the result as an operational estimate and reconciliation tool. Final commercial quantity may still follow contract terms, official documentation, and surveyor agreement.
Frequently asked questions
Is draught survey always exact?
No. It is an estimate based on measurements and corrections. Good practice can produce strong accuracy, but uncertainties remain.
Why is density correction so important?
Hydrostatic displacement tables are usually based on standard seawater density. If actual dock water is lighter or heavier, displacement must be corrected to avoid systematic quantity error.
Can this calculator replace a certified marine surveyor?
No. It is a practical tool for planning and verification, not a legal substitute for certified survey procedures.
Final thoughts
A disciplined draught survey calculation process can prevent disputes, improve cargo accountability, and strengthen voyage reporting. Combine this calculator with accurate readings, vessel-specific hydrostatic data, and transparent records for the most reliable outcome.