How this planner works (and what it does not model)
This calculator estimates the energy to heat the water in your tub, then converts that energy into
firewood required and heating time using your stove efficiency, wood heat content, and burn rate.
It is designed for planning questions such as: “How much wood should I stage next to the tub?” “How early do I need to start the fire?”
and “What will a month of weekend soaks cost if I buy wood by the cord?”
The estimate is intentionally straightforward. It treats the tub as a known volume of water and assumes the stove delivers a consistent
fraction of the wood’s heat into that water. In real use, performance varies: a tight-fitting insulated cover can dramatically reduce
evaporation losses, while wind and low circulation can make the same tub feel like it takes forever. The goal is not perfect prediction;
the goal is a repeatable baseline you can calibrate after one or two sessions.
Core formulas
The model uses the standard water-heating relationship: water needs about 1 BTU per pound per °F.
One gallon of water weighs about 8.34 lb.
- Water mass (lb) =
volume_gal × 8.34
- Temperature rise (°F) =
target_temp − start_temp
- Delivered energy to water (BTU) =
water_mass × temp_rise
- Wood energy required (BTU) =
delivered_energy ÷ efficiency
- Wood required (lb) =
wood_energy_required ÷ wood_btu_per_lb
- Heating time (hours) =
wood_required ÷ burn_rate_lb_per_hour
For reference, the energy balance can be written as:
Where V is gallons, Ti is starting temperature, Tf is target temperature,
and η is overall stove-to-water efficiency as a decimal (e.g., 45% → 0.45).
Worked example (using the default values)
Suppose you have a 450-gallon tub starting at 55°F and you want 103°F.
That’s a 48°F rise. Water mass is 450 × 8.34 ≈ 3,753 lb.
Delivered heat to the water is 3,753 × 48 ≈ 180,144 BTU.
With 45% efficiency, the stove must produce about 180,144 ÷ 0.45 ≈ 400,320 BTU.
With wood at 7,200 BTU/lb, that’s ≈ 55.6 lb of wood.
At a burn rate of 7.5 lb/hour, heating time is ≈ 7.4 hours.
Your real-world result can be lower or higher depending on cover use, wind, water circulation, and wood moisture.
Use this example as a quick check that your inputs are in the right ballpark, then adjust efficiency or burn rate after you observe a real heat-up.
Assumptions & limitations
- Water heating focus: The estimate is based on heating the water mass. It does not explicitly add energy for warming the tub shell, stove body, benches, or plumbing.
- Heat loss not explicitly modeled: Wind, cold air, evaporation, rain/snow, and ground contact can materially increase wood use and time.
- Efficiency is an overall delivered fraction: “Stove efficiency” here means the fraction of wood energy that ends up in the water. It varies with stove design, draft, firing technique, and circulation.
- Wood BTU varies: Species and moisture content change BTU/lb. Wet/green wood can reduce effective output and slow heat-up.
- Burn rate is an average: Real burn rate changes during startup and as you approach target temperature.
- Safety: This tool does not enforce safe soaking temperatures. Follow manufacturer guidance and local safety recommendations.
Practical tips for better estimates
- Measure your starting temperature at the tub, not at the tap—hose runs and cold tubs can drop it.
- Use a cover during heat-up to reduce evaporation losses (often the biggest loss term).
- Calibrate once: After one real session, adjust efficiency or burn rate so the estimate matches your observed time/wood. Then reuse those calibrated values.
- Plan for weather: On windy or freezing nights, run a conservative scenario by lowering efficiency or increasing your expected time buffer.
FAQ
- How do I estimate my tub volume (gallons)?
-
Use the manufacturer rating if available. Otherwise approximate from dimensions and convert cubic feet to gallons
(1 ft³ ≈ 7.48 gal). For round tubs: volume ≈ π × radius² × water depth.
- What’s a reasonable stove efficiency to use?
-
Many setups land in a broad range (often 30%–70% delivered to the water). If you are unsure, start conservative
(lower efficiency) and then refine after a real heat-up.
- Does the cost estimate include kindling, starters, or your time?
- No. It estimates firewood cost only, based on your price per cord and cord weight.
Planning notes for wood-fired soaking tubs
Wood-fired hot tubs are simple in concept but variable in practice. Two sessions with the same tub can require very different
amounts of wood depending on wind, ambient temperature, whether you use a cover, and how dry your wood is.
Use this planner to create a baseline, then refine it with your own observations.
If your real heat-up time is consistently longer than the estimate, the most common causes are evaporation
(no cover), low circulation through the heater, and wet wood. If it is consistently shorter,
your effective efficiency may be higher than you assumed, or your burn rate may be higher than your average estimate.
For repeat users who keep water between sessions, you can treat the starting temperature as the cooled-down temperature
at the beginning of the next session. That often reduces the temperature rise dramatically and can cut both wood and time.
How to choose realistic inputs (quick guidance)
The most useful results come from inputs that match how you actually operate your tub. If you are new to wood-fired heating,
start with conservative assumptions and then tighten them after you gather a little data. The checklist below helps you avoid
the most common planning mistakes.
- Volume: If your tub is not filled to the brim, use the typical fill level. A 10% volume error becomes a 10% energy error.
- Starting temperature: Cold tubs can pull heat from the water during the first hour. Measuring at the tub after filling is more accurate than using the tap temperature.
- Target temperature: If you often overshoot and then add cold water, consider setting a slightly lower target and planning a “top-up” step.
- Efficiency: Treat this as “delivered to water.” External stoves, long plumbing runs, and poor circulation reduce it. Submerged stoves and good mixing increase it.
- Wood BTU/lb: Species matters, but moisture matters more. If your wood hisses, steams, or feels heavy, use a lower BTU/lb and expect longer heat-up.
- Burn rate: Use an average you can sustain. A short burst of high burn rate at startup does not mean you can keep that pace for six hours.
Scenario planning: build a time buffer you can live with
A practical way to use this page is to run two scenarios: a baseline and a conservative case. Keep your tub volume and temperatures the same,
then lower efficiency (or lower wood BTU/lb) to represent wind, colder air, or damp wood. The difference between the two heating times is your
planning buffer. If the conservative case says you need an extra hour, you can decide whether to start earlier, use a cover, stage more wood,
or accept a later soak.
If you host guests, the buffer matters more than the exact number. People remember waiting around in robes more than they remember whether you
burned 50 lb or 60 lb of wood. A reliable routine is: stage the full estimated wood, light early, cover the tub during heat-up, and mix the water
periodically so the thermometer reflects the average temperature rather than a warm surface layer.
Interpreting the outputs (what each number is telling you)
The results include energy (BTU), wood required (lb), and heating time (hours). Energy is the “physics” number: it scales with gallons and temperature rise.
Wood required converts that energy into fuel using your efficiency and wood heat content. Heating time converts fuel into a schedule using your burn rate.
If one output looks wrong, trace it back: a surprisingly long time usually comes from a low burn rate or low efficiency; a surprisingly high wood requirement
often comes from a large temperature rise or low BTU/lb.
The monthly cost estimate uses your sessions per month and your wood price per cord. If you cut your own wood, you can still use the cost section by entering
an “all-in” value that reflects chainsaw fuel, splitter wear, permits, delivery, or your time. Even a rough cost helps you compare wood-fired heating to other
options and decide how much wood to store before the season.
Safety and operating notes (non-math, but important)
Wood-fired tubs involve open flame, hot metal, and very hot water. Keep combustibles away from the stove and chimney, follow the stove manufacturer’s clearances,
and use a thermometer you trust. Avoid overheating: very hot water can be dangerous, especially for children, older adults, and anyone with health conditions.
If you are unsure about safe temperatures or exposure time, consult local guidance and your healthcare professional.
Finally, treat this planner as a starting point. The best way to improve accuracy is to record one real session: note the starting temperature, ambient conditions,
wood species and moisture, how many pounds you actually burned, and how long it took to reach target. Then adjust the efficiency or burn rate inputs so the model
matches your observed outcome. After that calibration, the calculator becomes a reliable tool for your specific tub and stove.