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Hot Tub & Spa Volume Calculator

Estimate the total gallons of water in a round hot tub or spa, for chemical dosing.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
6.89 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Estimated spa volume

823.43 gallons

Check your inputs

This treats the spa as a simple cylinder — built-in seats, jets, and equipment displace some water, so actual capacity is typically 5-15% less than this raw geometric estimate. Check your manufacturer's specified capacity for precise chemical dosing.

Volume in cubic feet
110.08 ft³

At the values currently entered, the estimated spa volume works out to 823 gallons. Confidence is moderate: the method is sound, but real materials and site conditions vary. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Round vessel volume (gallons) = pi x radius² x depth (all in ft) x 7.48 gallons per cubic ft

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Why is the manufacturer's stated capacity usually different from this estimate?
Built-in seats, headrests, jets, and internal equipment all take up space that would otherwise hold water, so a manufacturer's tested capacity is typically lower than the raw cylinder volume this calculator computes.
Does this work for square or rectangular hot tubs?
No — this is for round or octagonal tubs approximated as a cylinder. A square or rectangular spa should use the same length x width x depth x 7.48 formula as the Pool Volume Calculator instead.
Why does accurate volume matter for a hot tub specifically?
Hot tubs have a much smaller water volume than pools, so chemical dosing errors have a proportionally bigger impact on water balance — getting the volume right matters more for a spa than for a large pool.