Water Purification Calculator

Calculate emergency water purification needs by household size, duration, and purification method.

About this calculator

Emergency water planning splits into two separate questions this calculator keeps distinct: how much total water do you need, and how much purification supply does it take to make that water safe to use. Total Water Needed is driven entirely by Number of People, Duration (days), and your two per-person daily allowances — commonly cited emergency-preparedness guidance suggests roughly a gallon per person per day for drinking, with a comparable additional amount for sanitation and hygiene needs, so this calculator lets you set both independently rather than assuming a single fixed figure. Purification Method has no effect on that total — it only determines how much of the chosen supply (boiling time, bleach volume, filter-minutes, tablet count, or UV treatments) is needed to treat the same fixed quantity of water.

Each method uses commonly cited approximate figures: bleach dosing at 8 drops per gallon of unscented sodium hypochlorite, one purification tablet per liter, and a portable filter or UV pen's typical throughput — treat these as planning-level references rather than guaranteed performance for a specific product, and always follow the manufacturer's stated dosing and contact-time instructions for your actual bleach concentration and equipment. This tool does not verify that a given method achieves any specific pathogen log-reduction standard; boiling for a full rolling minute (three minutes above roughly 6,500 feet elevation) remains one of the most broadly reliable methods when supply logistics allow it.

Inputs

Number of people

Duration (days)

Drinking water (gal/person/day)

Sanitation water (gal/person/day)

Purification method

Tab to move between fields

Results

Total water needed (gal)

Total water needed (L)211.96

Daily drinking water (gal)4

Daily total water (gal)8

Purification supply needed0.02

5-gal containers needed12

Supply DescriptionLiters of 8.25% unscented sodium hypochlorite bleach

Supply LabelBleach needed (liters)

How to Use This Calculator

Enter Number of People and Duration (days) of the emergency event.
Set Drinking Water (gal/person/day) — minimum is 0.5 gallons, 1 gallon recommended.
Enter Sanitation Water (gal/person/day) for hygiene and cooking.
Select Purification Method (boiling, chlorine tablets, filter, UV, or mixed).
Review Total Water Needed (gallons and liters) and purification material quantities needed.

How the result changes with Number of people

Number of people	Total water needed (gal)
2	28
3	42
6	84
10	140

What each input means

Number of people: Number of people who need purified water.
Duration (days): Number of days water purification is needed.
Drinking water (gal/person/day): FEMA/Ready.gov's baseline is ~1 gal/person/day combined for drinking AND sanitation together, split roughly in half; double drinking alone in hot climates or with heavy activity.
Sanitation water (gal/person/day): Water for hygiene, cooking, and sanitation — part of FEMA/Ready.gov's ~1 gal/person/day combined baseline, not a separate 1-gal minimum of its own.
Purification method: Method determines the supply quantity needed, not the total water required.

What each result means

Total water needed (gal): Total gallons of purified water needed for the full duration.
Total water needed (L): Total liters of purified water needed.
Daily drinking water (gal): Gallons of drinking water needed per day for everyone.
Daily total water (gal): Daily total including drinking and sanitation.
Purification supply needed: Amount of purification supply (bleach, tablets, time, etc.) based on selected method.
5-gal containers needed: Number of standard 5-gallon water containers for storage.

How this is calculated

Worked example, using the default values

Identify Input Parameters
5 parameters
Number of people = 4, Duration (days) = 7, Drinking water (gal/person/day) = 1, Sanitation water (gal/person/day) = 1, Purification method = 1 = 5 input(s) provided
Calculate Total water needed
Total water needed = dailyTotalGal * durationDays
56 = 56
Calculate Total water needed
Total water needed = totalGallonsNeeded * 3.785
211.96 = 211.96
Calculate Daily drinking water
Daily drinking water = people * drinkingGalPerPersonDay
4 = 4

Engine last updated August 26, 2026. Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Does changing the purification method change how much water I need?

No — Total Water Needed is set entirely by Number of People, Duration, and your drinking and sanitation allowances per person per day. Purification Method only changes how much of the chosen treatment supply (boiling minutes, bleach volume, tablets, or filter/UV time) it takes to treat that same fixed quantity of water.

Why does the calculator separate drinking water from sanitation water?

Emergency water planning guidance commonly distinguishes drinking needs from water used for hygiene, food preparation, and sanitation, since both matter but serve different purposes and can be adjusted independently — heat, physical activity, medical conditions, and children or nursing mothers can all raise drinking needs above the baseline. Splitting them lets you scale each based on your household's actual situation rather than one blended number.

How reliable is boiling compared to chemical or filter methods?

Boiling at a full rolling boil for at least one minute (three minutes above roughly 6,500 feet elevation, where water boils at a lower temperature) is widely regarded as one of the most reliable purification methods when fuel and time allow, since it doesn't depend on precise chemical dosing or filter pore size holding up over time. Bleach, tablets, and filters are convenient alternatives, but each depends on correct dosing, contact time, or maintained equipment to perform as expected.

Is the bleach dosing figure exact for any bleach I have on hand?

The 8-drops-per-gallon figure assumes unscented, plain sodium hypochlorite bleach at a specific concentration (commonly cited around 8.25%), and dosing needs to change if your bleach's concentration differs or if it contains added scents, thickeners, or other additives not meant for water treatment. Always check your specific product's label and use bleach labeled safe for water disinfection.

Why does the calculator ask for a specific duration in days?

Duration (days) multiplies directly into Total Water Needed, since the daily drinking-and-sanitation requirement compounds over however long the emergency situation is expected to last. Underestimating duration is a common planning error, so many preparedness guides recommend planning for at least a multi-day supply rather than assuming a short-term disruption.

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

Military Planning