Water calculators you can trust.
160+ free tools for the weight, flow, pressure, cost, treatment and science of water — each one on a real formula, verified number-by-number, and reviewed by a licensed expert.
Water shows up in almost every practical problem around a home, a job site, or a lab bench — and nearly all of it comes down to a calculation. How heavy is a full tank before you put it on a shelf? What flow rate does that pipe actually carry? How big a softener does your water hardness call for, and what will the running cost be over a year? Water Calculator brings all of those questions together in one place, with a tool for each and a plain-language explanation of the reasoning behind it.
The library covers eight areas — the science of water (density, weight, heat, viscosity and phase behaviour), flow and pressure through pipes and channels, everyday usage and cost at home, water treatment and chemistry, and the pumps and drainage that move it all. Each calculator is a single, focused tool: enter what you know, read the answer instantly in the units you want, and see the formula, the assumptions, and the sources it rests on. Nothing is hidden behind a sign-up, and nothing you type ever leaves your browser.
Browse by category
All calculators →Water Science
Density, weight, heat, viscosity, humidity and more.
29 calculators →Flow
Flow rate, velocity, friction loss, weirs and fixtures.
25 calculators →Pressure
Pressure at depth, PSI, head conversions and losses.
15 calculators →Usage
Household and per-person use, footprints and savings.
23 calculators →Cost
Water bills, rates, leak losses and heating costs.
13 calculators →Treatment
Softening, filtration, RO, UV and water chemistry.
22 calculators →Pumps
Sizing, head, horsepower, efficiency and NPSH.
17 calculators →Drainage
Pipe slope and sizing, sumps, gutters and stormwater.
16 calculators →See one in action
Every tool works like this — type a value and read the answer instantly, with the working and sources on the full page.
How much does water weigh?
At room temperature (20 °C). Change the volume or unit.
Popular calculators
See all →Why Water Calculator
Most “water calculator” pages online are thin — one box, a guessed number, a wall of ads. We built the opposite.
Real formulas, not lookups
The same standards engineers use — Kell, Manning, Hazen–Williams, Langelier — implemented at full precision with exact constants.
Every number verified
Each figure in a table or example is checked by hand against the live tool. If they disagree, the page doesn't ship.
Cited & expert-reviewed
Built from primary sources — EPA, USGS, NIST, AWWA — and reviewed by credentialed water professionals who are named on each page.
Free & private
No account, no paywall. Calculators run in your browser, so the values you enter are never sent to us or stored.
Read how we work in our methodology and editorial policy.
The people behind the numbers
Meet the team →
Texas-licensed Professional Engineer, 13+ years and 180+ projects in residential water systems. Leads the engineering and hydraulics tools — science, flow, pressure, pumps and drainage.
Environmental scientist (MS, Water Resources) with 9+ years and 150+ assessments. Leads water quality, everyday usage and long-term cost — treatment, usage and cost.
Who it's for
Homeowners & DIYers
Straight answers — how heavy is a full tank, what will this bill be, is my water hard?
Trades & pros
Fast, defensible field calculations for plumbers, installers and contractors.
Students & educators
Worked formulas and clear explanations that show the method, not just the result.
Engineers
Reference-grade tools on the standard equations, with sources and assumptions stated.
Understanding water calculations
A few ideas come up again and again across these tools. Getting them right is what separates a dependable number from a rough guess.
Weight, volume and density
Most water questions reduce to the link between how much water you have and how much it weighs — and density is the bridge. A US gallon weighs about 8.34 lb, a liter almost exactly 1 kg, and a cubic foot roughly 62.4 lb. Those figures shift slightly with temperature, because water expands as it warms and is densest at about 4 °C.
Our weight, density and buoyancy tools all rest on this relationship.
Flow, pressure and head
Move water and two things matter: how fast it goes and how hard it pushes. Flow rate ties velocity to pipe size; pressure and “head” are two ways of describing the same push, where every foot of vertical height adds about 0.43 psi. Friction in pipes and fittings steadily eats into both.
Tools like pipe flow, pressure at depth and friction loss use the Hazen–Williams, Darcy–Weisbach and Manning equations engineers rely on.
Usage and cost
A typical household uses around 300 gallons a day, with toilets, showers, faucets and the washing machine accounting for most indoor use. Turning that into a bill means layering on your utility's rate — often tiered — plus any sewer charge and the energy cost of heating.
Estimate your own with the household usage and water bill calculators.
Treatment and chemistry
Hardness, measured in grains per gallon (1 gpg = 17.1 mg/L as calcium carbonate), sets how large a softener you need and how often it regenerates. Indices like the Langelier Saturation Index predict whether water will scale up or corrode your pipes.
See the softener sizing, grains-per-gallon and Langelier index tools.
Common questions
Are the calculators free?
Yes — every calculator is completely free, with no account or sign-up, and each runs entirely in your browser.
How accurate are they?
Each tool is built on the recognised standard formula for the quantity — the Kell equation for density, Manning's for channels, Hazen–Williams for pipe flow, and so on — with exact constants and every figure verified against the live tool. Each page lists its sources.
Who writes and reviews the calculators?
Credentialed water professionals, assigned by expertise: John Mac, PE leads the engineering and hydraulics tools, and Sarah Bennett, MS leads water quality, usage and cost. Every page carries the name of the person responsible for it.
How much does a gallon of water weigh?
About 8.34 pounds (3.79 kg) at room temperature. Try the Water Weight Calculator for any volume and temperature.
Do you store the values I enter?
No. The calculators run entirely in your browser, so nothing you type is sent to us or stored. See our privacy policy for details.
Find your calculator
160+ free water tools, from lab-grade science to the plumbing and bills of an ordinary home.
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