LNG Tank Boil-off Calculator

Inputs

Use commas in numbers if needed. Units are shown.

White theme • Responsive grid

Tank volume (m³) *

Gross tank capacity.

Fill level (%) *

Used to estimate inventory mass.

LNG density (kg/m³) *

Typical range 430–470 kg/m³.

Calculation method

Choose one approach, then fill related fields.

Days in period *

Totals are multiplied by this duration.

Latent heat of vaporization (kJ/kg) *

Used for heat-balance conversion.

Heat ingress (kW)

If set, overrides U·A·ΔT inputs.

Surface area (m²)

External tank area for heat transfer estimate.

Overall U-value (W/m²·K)

Lower values mean better insulation.

Temperature difference ΔT (K)

Ambient-to-cryogenic driving temperature.

Extra heat load (kW)

Pumps, piping, agitation, and other sources.

Boil-off rate (%/day)

Used only for inventory-loss method.

Reliquefaction capacity (kg/day)

BOG mass handled by a re-liquefaction unit.

Additional recovery (%)

Extra capture to fuel or process streams.

LHV for energy reporting (MJ/kg)

Used to convert mass boil-off into energy.

Example data table

Example values are illustrative and should be verified for your tank design.

Scenario	Tank volume (m³)	Fill (%)	Density (kg/m³)	Heat ingress (kW)	Latent heat (kJ/kg)	BOG (kg/day)
Large full tank	160,000	85	450	35	510	5,929
Moderate heat gain	50,000	70	440	18	510	3,051
Inventory-loss method	20,000	60	460	—	510	3,312

Formula used

1) Heat ingress method

Estimate heat gain using either a direct value, or a heat transfer model:

Q̇ (kW) = (U · A · ΔT)/1000 + Q̇_extra
BOG (kg/day) = (Q̇ · 86400) / L

Where U is W/m²·K, A is m², ΔT is K, and L is kJ/kg.

2) Inventory-loss (%/day) method

Convert an assumed boil-off fraction to mass generation:

Inventory mass (kg) = V_tank · (Fill/100) · ρ
BOG (kg/day) = Inventory mass · (BOR/100)
Implied Q̇ (kW) = (BOG · L) / 86400

This is useful when BOR is known from operations data.

Recovery and venting

Net venting accounts for handling capacity:

Handled = min(BOG, Reliq + BOG·Recovery%)
Net vented = max(0, BOG − Handled)

How to use this calculator

Enter tank volume, fill level, density, and the analysis duration.
Select a method: heat ingress or inventory-loss based.
If using heat ingress, provide Q̇ directly, or enter U, area, and ΔT.
If using inventory-loss, enter a realistic BOR (%/day) from records.
Add reliquefaction and recovery values to estimate net venting.
Click Calculate. Download CSV or PDF for documentation.

Boil-off drivers in full-containment tanks

Boil-off gas forms when heat leaks through insulation, penetrations, and piping connections. Even small heat rates become large daily vapor generation because latent heat at cryogenic temperature is finite. The calculator converts heat ingress to mass flow, helping construction teams compare insulation quality, roof details, and foundation interfaces during commissioning. It also helps prioritize repairs on cold bridges, vapor barriers, and valve boxes that often drive unexpected losses.

Selecting heat-ingress inputs for design checks

Use the heat method when you can estimate overall U-value, exposed surface area, and the ambient-to-cryogenic temperature difference. When detailed thermal modeling is not available, start with conservative U-values from specifications, then refine using site measurements and vendor test data. Extra heat loads represent pumps, recirculation, and warm return streams. Treat U as an “as-built” value and update it after insulation surveys or infrared inspections.

Using inventory-loss rates for operations forecasting

Use the inventory-loss method when operations provide a verified percent-per-day loss. This is common for steady storage with stable weather and known tank performance. The tool converts the percentage into daily mass and also reports the implied heat ingress, allowing a quick comparison against the original design basis and insulation assumptions. Trending effective boil-off can reveal drifting vacuum quality, damaged perlite, or changes in roof seal integrity.

Interpreting vented versus handled boil-off

Boil-off is not always wasted. Many sites recover vapor as fuel, send it to process users, or reliquefy it. Enter reliquefaction capacity and recovery percentage to estimate net venting. Net venting highlights flare loading, relief system sizing checks, and emissions reporting boundaries that may apply during construction and early operation. Use the energy-equivalent output to communicate impacts in familiar fuel terms to stakeholders.

Documentation and construction-phase considerations

Record assumptions for density, latent heat, and heating value because LNG composition varies by supply. During construction, temporary piping, incomplete insulation, and frequent access can increase heat gain. Use exported reports to track changes between milestones, justify corrective actions, and align handover documentation with owner requirements, safety reviews, and acceptance testing. Keep a clear audit trail of input sources, instrument calibration dates, and any applied safety factors.

FAQs

Quick answers for common field questions.

Q1. Which method should I use for planning?

Use heat ingress when you have U, area, and temperature difference, or a measured heat rate. Use inventory-loss when you have a verified percent-per-day loss from operations or commissioning data.

Q2. What LNG density should I enter?

Use the site density from custody transfer or composition calculations when available. If unknown, start around 430–470 kg/m³ and run a sensitivity check. Density affects liquid volume equivalents and inventory mass.

Q3. Why is latent heat important?

Latent heat links heat gain to vapor generation. Higher latent heat means less mass boils off for the same heat ingress. Use values provided by your LNG property package or vendor data when possible.

Q4. How do I estimate a realistic U-value?

Start with the insulation specification and expected performance, then adjust for penetrations, settlement, and moisture. For upgrades, compare calculated implied heat ingress against measured boil-off to back-calculate an effective U.

Q5. How is net venting calculated?

The tool subtracts handled vapor from generated boil-off. Handled vapor includes a fixed reliquefaction capacity plus a recovery percentage of the remaining boil-off. Net venting cannot be negative; it bottoms at zero.

Q6. What does the energy equivalent output mean?

It converts daily boil-off mass into an approximate fuel energy using the entered heating value. This helps compare losses to burner demand, generator fuel use, or contractual fuel gas allocations.