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Marine Holding Tank Calculator

Size your holding tank and plan pump-out schedules

Units

Required Tank Capacity

11

gallons (including 10% safety margin)

Daily Usage

2 gal/day

Trip Total

10 gal

Tank Sufficient

Fill Level After Trip40%

Usage Schedule

Days of Capacity

12.5

Days Before Pump-out

11.3

Flush Water Usage

Per Day (est. 5 flushes/person)10 gal
Total Trip50 gal

Total Waste by Crew Size

2 crew5 gal
4 crew10 gal
6 crew15 gal
8 crew20 gal

Holding Tank Sizing Guide by Boat Length

Boat LengthTypical TankCrew of 2Crew of 4Crew of 6
22-26 ft10-15 gal20 days10 days6.7 days
27-32 ft20-30 gal40 days20 days13 days
33-40 ft30-50 gal60 days30 days20 days
41-50 ft50-80 gal100 days50 days33 days
50+ ft80-120 gal160 days80 days53 days

What You'll Need

Star Brite Ultimate Aluminum Cleaner & Restorer 64oz

$18-$254.5
View on Amazon

3M Marine Adhesive Sealant 5200 Fast Cure White 3oz

$14-$204.5
View on Amazon

Shoreline Marine Bilge Pump 600 GPH 12V

$15-$254.3
View on Amazon

Star Brite Ultimate Aluminum Cleaner & Restorer 64oz

$18-$254.5
View on Amazon

3M Marine Adhesive Sealant 5200 Fast Cure White 3oz

$14-$204.5
View on Amazon

Shoreline Marine Bilge Pump 600 GPH 12V

$15-$254.3
View on Amazon

As an Amazon Associate, we earn from qualifying purchases.

Frequently Asked Questions

Q

How big should a boat holding tank be?

Tank size depends on crew count, trip duration, and usage rate. At the standard rate of 0.5 gallons per person per day, a crew of 4 on a 5-day trip needs about 11 gallons (including 10% safety margin). Most cruising boats carry 20–40 gallon tanks.

  • Standard usage rate: 0.5 gallons per person per day
  • Formula: crew × rate × days × 1.1 (10% safety margin)
  • 4 crew, 5 days = 4 × 0.5 × 5 × 1.1 = 11 gallons minimum
  • Weekend boat (22–26 ft): 10–15 gallon tank is typical
  • Cruising boat (33–40 ft): 30–50 gallon tank is typical
Crew Size3-Day Trip7-Day Trip14-Day Trip
2 people3.3 gal7.7 gal15.4 gal
4 people6.6 gal15.4 gal30.8 gal
6 people9.9 gal23.1 gal46.2 gal
8 people13.2 gal30.8 gal61.6 gal
Q

How often do you need to pump out a holding tank?

Pump-out frequency depends on tank size and usage. A 25-gallon tank with 4 crew at standard usage (0.5 gal/person/day) should be pumped when it reaches 90% capacity, which is about every 11 days. Plan pump-outs at marinas along your route.

  • Pump out at 90% capacity — never let the tank reach 100%
  • Days before pump-out = (tank size × 0.90) / (crew × daily rate)
  • 25-gal tank, 4 crew: (25 × 0.90) / (4 × 0.5) = 11.25 days
  • Most marinas charge $5–15 per pump-out
  • Some states require pump-out within 3 miles of shore (no-discharge zones)
Q

What type of marine head uses the least water?

Vacuum heads use the least water at approximately 0.15 gallons per flush. Composting heads use no water at all but require a separate system. Electric heads use about 0.3 gallons per flush, and manual heads use about 0.5 gallons per flush.

  • Composting head: 0 gallons per flush (no water required)
  • Vacuum head: 0.15 gallons per flush (most water-efficient flushing head)
  • Electric head: 0.3 gallons per flush (standard for modern boats)
  • Manual head: 0.5 gallons per flush (most common, most water use)
  • Switching from manual to vacuum saves ~70% of flush water
Head TypeWater/FlushDaily (5 flushes)Cost Range
Manual0.50 gal2.5 gal$200–$500
Electric0.30 gal1.5 gal$500–$1,500
Vacuum0.15 gal0.75 gal$1,000–$2,500
Composting0 gal0 gal$1,500–$3,000
Q

Can I discharge a holding tank in the ocean?

Federal law prohibits discharge of untreated sewage within 3 nautical miles of shore. Beyond 3 miles, treated waste from a Type I or II MSD may be discharged. Some states and bodies of water are designated no-discharge zones where all discharge is prohibited regardless of distance.

  • Within 3 nm: no discharge of any sewage (treated or untreated)
  • Beyond 3 nm: treated sewage from MSD Type I/II may be discharged
  • No-discharge zones (NDZ): zero discharge regardless of distance
  • Great Lakes, many bays, and harbors are designated NDZs
  • Fines for illegal discharge: $2,000–$25,000 per violation
Q

How do I calculate flush water usage separately?

Flush water depends on the head type and number of uses per person per day. At an average of 5 flushes per person per day, a manual head (0.5 gal/flush) uses 2.5 gallons per person daily. For 4 crew over 5 days, that is 50 gallons of flush water from the freshwater tank.

  • Average flushes per person per day: 5
  • Manual head: 5 × 0.5 gal = 2.5 gal/person/day flush water
  • Electric head: 5 × 0.3 gal = 1.5 gal/person/day flush water
  • Vacuum head: 5 × 0.15 gal = 0.75 gal/person/day flush water
  • 4 crew, 5 days, manual head = 4 × 2.5 × 5 = 50 gal from freshwater tank

Example Calculations

1Weekend Cruise: 4 Crew, 5 Days

Inputs

Crew Size4
Trip Duration5 days
Usage Rate0.5 gal/person/day
Head TypeManual
Tank Capacity25 gallons

Result

Required Capacity11.0 gallons
Daily Usage2.0 gal/day
Days of Capacity12.5 days
Pump-outs Needed0

Daily = 4 × 0.5 = 2.0 gal/day. Trip total = 2.0 × 5 = 10.0 gal. Required = 10.0 × 1.1 = 11.0 gal. Days of capacity = 25 / 2.0 = 12.5 days. Tank is sufficient for the trip.

2Extended Cruise: 6 Crew, 14 Days

Inputs

Crew Size6
Trip Duration14 days
Usage Rate0.5 gal/person/day
Head TypeElectric
Tank Capacity40 gallons

Result

Required Capacity46.2 gallons
Daily Usage3.0 gal/day
Days of Capacity13.3 days
Pump-outs Needed1

Daily = 6 × 0.5 = 3.0 gal/day. Trip total = 3.0 × 14 = 42.0 gal. Required = 42.0 × 1.1 = 46.2 gal. Days before pump-out = (40 × 0.90) / 3.0 = 12.0 days. One pump-out needed.

3Liveaboard: 2 Crew, 30 Days

Inputs

Crew Size2
Trip Duration30 days
Usage Rate0.75 gal/person/day
Head TypeVacuum
Tank Capacity50 gallons

Result

Required Capacity49.5 gallons
Daily Usage1.5 gal/day
Days of Capacity33.3 days
Pump-outs Needed0

Daily = 2 × 0.75 = 1.5 gal/day. Trip total = 1.5 × 30 = 45.0 gal. Required = 45.0 × 1.1 = 49.5 gal. Days of capacity = 50 / 1.5 = 33.3 days. Tank is barely sufficient.

Formulas Used

Required Tank Capacity

Capacity = Crew × Usage Rate × Days × 1.10

Calculates minimum holding tank size with a 10% safety margin.

Where:

Crew= Number of people aboard
Usage Rate= Gallons of waste per person per day (standard: 0.5)
Days= Trip duration in days
1.10= 10% safety margin multiplier

Days of Capacity

Days = Tank Size / (Crew × Usage Rate)

Calculates how many days the tank will last before needing pump-out.

Where:

Tank Size= Holding tank capacity in gallons
Crew= Number of people aboard
Usage Rate= Gallons of waste per person per day

Flush Water Usage

Flush Water = Crew × Flushes/Day × Flush Volume × Days

Estimates total freshwater consumed by the marine head during the trip.

Where:

Crew= Number of people aboard
Flushes/Day= Average flushes per person per day (typically 5)
Flush Volume= Gallons per flush (0.15–0.5 depending on head type)
Days= Trip duration in days

Marine Holding Tank Sizing and Pump-Out Planning

Every boat with an installed head must have a holding tank to comply with federal and state regulations. Sizing the tank correctly means balancing capacity against available space and weight. Too small and you’re making frequent pump-out stops; too large and you’re carrying unnecessary weight and losing storage space.

The standard usage rate of 0.5 gallons per person per day accounts for both waste and flush water entering the holding tank. This rate assumes a manual marine head with typical use. Switching to a vacuum head can reduce this to 0.3 gallons per person per day, significantly extending time between pump-outs.

Planning pump-out stops is essential for multi-day cruising. Use the 90% rule — never fill the tank beyond 90% to prevent overflow and sensor inaccuracy. Map marina pump-out stations along your route and schedule stops every 8–12 days for a typical 25-gallon tank with 4 crew aboard.

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Last Updated: Mar 25, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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