How Deep Should a Custom Inground Spa Be?
A spa can have enough floor depth and still put the water at the wrong height on a seated person. Choose the water depth above each finished seat for its intended users, then coordinate that seat with the footwell and operating waterline.
Aquatic Mechanical Engineering’s Sheet P-7 uses a 36-inch maximum-depth value and an 18-inch seating level in one published example. Confirm the measuring datum before comparing those values with your own drawing; they are not a universal recommendation or a local code limit. A shoulder-height soak depends on the seated person and the water above their seat; a single overall depth cannot establish that fit. Use the measurements and blank schedule below to compare proposed sections before construction. Read the designer’s original example.
Which inputs matter before choosing a depth?
Request a section through each different seat, using the proposed finished surfaces and the same waterline datum. “Spa depth” on a plan is too ambiguous to approve on its own.
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| Input or item | Value or source | Why it matters |
|---|---|---|
| Waterline to floor | Designer’s finished section | Defines total water depth in the footwell |
| Waterline to each seat | Seat-by-seat dimension | Defines water above the seated support surface |
| Seat to foot support | Finished section and user observation | Records where the feet can rest |
| Seat width and front-to-back depth | Dimensioned plan plus section | Keeps horizontal seat size distinct from immersion |
| Intended users and posture | Separate observation for each person | Avoids designing only for the tallest user |
Separate floor depth, seat immersion and bench size
Total water depth is the vertical distance from the stated operating waterline to the finished floor. Seat immersion is the vertical distance from that same waterline to the finished seat top. Bench depth often means the horizontal distance from front edge to backrest; ask which meaning a drawing uses before comparing numbers.
Spectrum Aquatics’ in-ground spa measuring guide lists water draft, top-of-spa to bench, total depth, bench width and wall slope separately. That manufacturer form is useful because it requires measured geometry instead of an undifferentiated depth number. It supplies no ideal soaking depth. Open the original measuring guide (PDF).
Coping height is another datum. If a dimension starts at the top of coping rather than the waterline, it includes the gap above the water. Ask the designer to identify both levels and the finish build-up; do not copy a shell-stage measurement into the finished-seat schedule.
Use the same datum to calculate water above a seat
For a level seat and the corresponding floor level, water above seat = waterline-to-floor depth − floor-to-seat height. Both dimensions must be vertical and refer to the same finished section. For a sloping seat or floor, specify the exact measuring point.
Hypothetical example: a floor 36 inches below the waterline and a seat 20 inches above that floor leave 16 inches of water above the seat (36 − 20 = 16). These are arithmetic inputs, not recommended construction dimensions. Lowering the seat increases immersion at the same waterline; deepening only the footwell leaves seat immersion unchanged.
A useful drawing revision says which surface moves and which level stays fixed. “Make the spa deeper” does not tell the builder whether to change the floor, the bench or the waterline.
Check shoulder immersion without sacrificing support
Compare proposed seat options for the people who will actually use them. Record the desired water contact at the chest or shoulders, the resting back position, foot support and the ability to approach and leave the seat. Standing height alone is not that record.
Our editorial recommendation is to retain a shallower seat option when users want different immersion levels, and compare it with the deeper option before fixing the shell. A dry mock-up can reveal reach and support questions, but it cannot establish underwater comfort. Keep those observations open until a suitable in-water trial or commissioning check is possible.
If a user lifts away from the seat or has to brace to remain in position, record the seat, waterline, posture and operating jet/air settings. Compare observations under the installer’s commissioning procedure. Do not assume deeper water will hold the person down, and do not improvise restraints or change suction fittings.
Treat neck-jet position as a separate design check
A preferred waterline does not establish a neck-jet installation height or a therapy result. Waterway’s manufacturer FAQ explicitly says the jet type is needed to determine its flow. Obtain the selected fitting’s current installation requirements and have its location checked against the intended seated posture. Read Waterway’s jet FAQ.
Once the depth schedule is clear, use the seat-fit and jet-contact review to work through backrest contour, contact zones and controls. This page concerns recreational immersion preferences; it does not prescribe treatment for neck or shoulder pain.
Record the decision in a depth schedule
Copy this blank schedule once for each different seat. Keep the section reference with every dimension; leave unknowns blank until the designer or the relevant observation supplies them.
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| Record | Measurement or evidence needed | Project value |
|---|---|---|
| Seat and section reference | Seat identifier, drawing revision and units | |
| Waterline datum | Operating mode and distance below coping | |
| Total water depth | Waterline to finished floor at stated point | |
| Seat immersion | Same waterline to finished seat top | |
| Seat and foot support | Floor-to-seat height, bench size, resting position | |
| User comparison | Desired immersion, approach, support and unresolved trial | |
| Decision and follow-up | Accepted section, reviewer and commissioning checks |
Download the blank depth schedule (CSV)
Return the completed schedule with a marked section showing the finished waterline, seat and footwell. Ask for a revised section whenever those levels change; a bench label alone is not confirmation that the immersion preference has been met.
When a residential depth discussion is not enough
For a public or commercial spa, identify applicable requirements with the project designer and local authority before setting levels or access details. Where the ADA spa provisions apply, the U.S. Access Board lists a lift, transfer wall or transfer system as accessible entry options. Its pool sloped-entry provisions are not a spa bench-depth recommendation. Read the Access Board’s spa guidance.
Review spa access and tiered bench planning
For the wider layout, heating and cover brief, return to in-ground spa design.
Method and sources
Compiled from the original Spectrum Aquatics measuring form, Aquatic Mechanical Engineering design page, Waterway manufacturer FAQ and U.S. Access Board guidance linked above; opened 15 October 2026. The dimensional calculation is hypothetical, and the blank schedule is an editorial design-review aid. No site testing, personal wet-test results, universal comfort depth or medical benefit is claimed.