How to Size Your State Water Heater: Gallons, BTUs & Household Needs
Ready to stop guessing and finally pick the right-sized State water heater with confidence? I’ve got you. This is a hands-on, worksheet-style guide you can follow in 20–30 minutes. We’ll translate confusing specs—First Hour Rating (FHR), recovery rate, BTU input, and GPM—into plain-English steps and quick math you can actually use.
You’ll finish with:
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A peak-hour hot water estimate (so you know what FHR or GPM to target)
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A capacity pick (gallons for tank models, GPM for tankless)
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A recovery/BTU check (to make sure your heater catches up fast enough)
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A simple decision path for special cases like high-demand households, cold-climate inlets, and compact spaces
Along the way, I’ll share four research-backed insights about real hot water use and sizing—sprinkled in exactly where they matter most.
Step 1: Sketch your “peak hour”
Your water heater only needs to cover your busiest 60 minutes—not the entire day. That’s your peak hour.
Common draws (typical hot-side contributions)
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Standard shower (2.0–2.5 GPM): 12–15 gallons of hot water for a 7–8 minute shower
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Efficient shower (1.5–1.8 GPM): 8–11 gallons
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Tub fill: 15–25+ gallons (varies with tub size and personal comfort)
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Dishwasher: 1.5–2.0 gallons (some units pre-heat water)
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Clothes washer (warm/hot cycle): 2–10 gallons (varies widely by machine and setting)
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Handwashing/cooking cleanup: 1–3 gallons total in a busy hour
Pro tip: When in doubt, round each item up by 10–15%. Under-sizing hurts more than over-sizing.
Your quick worksheet
List everything that could overlap within the same 60 minutes:
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Showers: ____ people × ____ gallons = ______
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Tub fill: ____ events × ____ gallons = ______
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Dishwasher: ____ cycles × ____ gallons = ______
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Laundry: ____ warm/hot loads × ____ gallons = ______
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Sinks/cleanup: ____ minutes × ____ GPM × 0.7 (hot fraction) = ______
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Peak-hour total = ______ gallons
Target for tank units: your FHR should meet or exceed this peak-hour total.
Target for tankless: your simultaneous GPM must meet the concurrent hot-water draw at your desired temperature rise (we’ll size that in Step 3).
Research insight #1: Real homes don’t use hot water in a smooth “average” line; usage happens in short, intense bursts (morning/evening peaks). Modeling studies that build realistic draw schedules show how much timing matters when you size equipment. U.S. DOE
Step 2: Pick your path—tank or tankless
This guide works for both, but your sizing “north star” differs:
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Tank water heaters: Size to FHR ≥ your peak-hour gallons.
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Tankless water heaters: Size to GPM ≥ the sum of fixtures running at once, at your required temperature rise (ΔT).
If you’re still deciding between tank vs. tankless, a simple rule is:
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Many overlapping uses + you like a lower upfront cost → Tank (bigger FHR = more buffer).
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Tight spaces + you prefer efficiency + fewer simultaneous draws → Tankless (right-sized GPM).
Step 3: Calculate your temperature rise (ΔT)
ΔT = Desired outlet temp – incoming cold water temp.
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Typical shower setpoint: 104–110°F
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Winter inlet temperature can be as low as 35–50°F in cold climates, 55–70°F in warmer ones.
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If you want 110°F in the shower and your winter inlet is 50°F, then ΔT = 60°F.
Why ΔT matters:
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Tankless units produce less GPM as ΔT goes up.
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Tank units rely more on stored volume, but recovery still slows with bigger ΔT unless BTU input is higher.
Research insight #2: Field and lab work show residential hot water demand and wait times vary dramatically with distribution layout and behaviors; assuming a single “average” ΔT or a single draw profile can mislead sizing. In short: size to your coldest realistic ΔT and real draw patterns. Prepp.in
Step 4: Translate peak hour into FHR (for tank models)
FHR (First Hour Rating) is how much “hot” water a storage heater can deliver in a fully heated hour. It combines tank volume + recovery.
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If your peak hour total = 65 gallons, look for FHR ≥ 65.
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Two tanks with the same volume can have very different FHRs if their BTU input/recovery differs.
Fast estimate (family lifestyle examples):
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Couple, 1 bath: 1 shower + dishes + handwashing → ~30–45 gallons
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Family of 4, 2 baths: 2 back-to-back showers + dishwasher + quick cleanup → ~55–70 gallons
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Family of 5 with a tub: 2–3 showers + partial tub fill → ~70–90+ gallons
If you’re between sizes, go up one FHR notch. That extra buffer saves morning arguments and cold-finish showers.
Research insight #3: Studies and review papers consistently find domestic hot-water energy share and draw intensity can be substantial—often second only to space conditioning—so undersizing can magnify peak-time shortfalls and standby/efficiency tradeoffs. ResearchGate
Step 5: Check recovery rate and BTU input (tanks)
Recovery rate = how many gallons per hour a heater can re-heat from your inlet temp to the setpoint.
A useful back-of-napkin:
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If you want to recover 30 GPH at ΔT = 60°F:
BTU/hr ≈ 30 × 8.34 × 60 ≈ 15,012 BTU/hr -
Real storage heaters often list input (e.g., 34,000–75,000 BTU/hr for gas) and a rated recovery at a given ΔT.
How to use this:
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Confirm your peak-hour gallons (Step 1).
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Pick a tank whose FHR meets it (Step 4).
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Prefer models with higher BTU input if you want quicker rebounds between back-to-back showers.
Rule of thumb: If your family does “stacked” showers and starts laundry right after, you’ll feel the benefit of higher recovery every single week.
Step 6: Translate your simultaneous uses into GPM (for tankless)
Tankless sizing starts with the sum of fixtures likely running at the same time, adjusted for ΔT.
Typical GPM (hot side at common mixing temps)
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Efficient shower: 1.5–1.8 GPM
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Standard shower: 2.0–2.5 GPM
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Dishwasher: 1.5–2.0 GPM (often much less hot-side if machine heats)
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Kitchen sink: 1.0–1.5 GPM
Example: Two showers (2.0 GPM each) + kitchen sink (1.0 GPM) = ~5.0 GPM at your target ΔT.
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If ΔT = 60°F, pick a unit that delivers ≥5.0 GPM @ 60°F rise (manufacturer performance charts show GPM at several ΔTs).
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If you sometimes run three showers, target 6–7 GPM @ 60°F rise.
Cold-climate caution: Your winter ΔT might be 70°F or more. GPM drops as ΔT climbs—so size to winter reality, not summer.
Research insight #4: NREL’s hot-water event schedule modeling demonstrates how peaky, high-concurrency draws can be; relying only on daily totals underestimates instantaneous GPM needs that drive tankless sizing. NREL Docs
Step 7: Factor in household patterns (choose your scenario)
Scenario A: “Morning rush hour”
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2–3 showers within 30–40 minutes + dishwasher pre-rinse
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Tank: Aim for FHR 60–80 gallons and solid recovery (higher BTU).
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Tankless: Target GPM 5–6 @ winter ΔT if two showers often overlap.
Scenario B: “Evening tub time”
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1 shower + partial tub fill + kitchen cleanup
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Tank: FHR ≥ 60–70 gallons.
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Tankless: GPM 4–5 @ winter ΔT, with a short high-flow tub demand.
Scenario C: “Compact space + efficiency first”
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Small utility closet, 1–2 simultaneous fixtures max
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Tankless shines: size 3–4 GPM @ winter ΔT for one shower + sink.
Scenario D: “Big family, many bathrooms”
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3 showers can overlap; weekends are wild
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Tank: Either high-FHR storage or storage + recirculation.
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Tankless: Consider a higher-GPM unit or two units in parallel for comfort.
Step 8: Convert the math into a shortlist
When you browse models:
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Tank: Filter by FHR first, then capacity (gallons), then input BTU and warranty.
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Tankless: Filter by GPM at your ΔT (use the product’s performance chart), then check venting/electrical requirements and warranty.
Quality-of-life checks:
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Noise and clearance (especially for tight closets)
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Condensate and venting routes (gas tankless)
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Hard water? Plan on descaling schedule or add conditioning.
A quick “State BTU chart” primer (what those numbers mean)
You’ll see a range of BTU inputs (gas storage and tankless):
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30–40k BTU/hr: entry-level gas storage
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40–55k BTU/hr: mid-range, better recovery
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60–75k BTU/hr: high-recovery storage or light-duty commercial edges
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100k–199k BTU/hr (and up): common in tankless, enabling high GPM @ high ΔT
Higher BTU input = faster recovery (tanks) or more GPM at a given ΔT (tankless). Just ensure your gas line/electrical supports the model’s requirements.
Step 9: Safety & temperature sanity checks
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Setpoint: Many households land at 120°F to balance scald risk and hygiene.
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Mixing valves: Consider them if you keep the tank hotter for legionella control while delivering safe outlet temps.
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Pressure/expansion: Closed systems may need an expansion tank—ask your installer.
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Code & venting: Follow local code for combustion air, vent materials, clearances, and condensate handling.
Step 10: Your printable mini-worksheet
1) Peak hour inventory
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Showers: ___ × ___ gal = ___
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Tub fill: ___ events × ___ gal = ___
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Dishwasher: ___ cycles × ___ gal = ___
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Laundry (warm/hot): ___ loads × ___ gal = ___
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Sinks/cleanup: minutes ___ × GPM ___ × 0.7 = ___
Peak-hour total (gallons) = _______
2) Winter ΔT
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Desired outlet temp: ___°F
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Winter inlet estimate: ___°F
ΔT = _______ °F
3) Choose path
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Tank target FHR ≥ _______ gal
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Tankless target GPM ≥ (sum of simultaneous fixtures) _______ @ ΔT _______°F
4) Shortlist
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Model A: FHR/GPM ___ | BTU input ___ | Fuel ___ | Warranty ___
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Model B: FHR/GPM ___ | BTU input ___ | Fuel ___ | Warranty ___
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Model C: FHR/GPM ___ | BTU input ___ | Fuel ___ | Warranty ___
5) Installation notes
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Vent route / electrical panel / gas line verified? Yes ☐ No ☐
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Hard water plan (descale/conditioner)? Yes ☐ No ☐
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Space/clearances confirmed? Yes ☐ No ☐
Worked examples (so you can sanity-check your numbers)
Example 1: Family of four, 2 baths, morning focus (tank)
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Peak hour: Two 8-min showers (2.0 GPM each ≈ 16–20 gal each), dishwasher pre-rinse (≈ 1–2 gal hot), handwashing (≈ 2 gal)
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Peak total ≈ 40–44 gallons (round to 50 for buffer)
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Target: FHR ≥ 50–60 gallons
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Why not a smaller tank? Because a 40-gal unit with weak recovery might lag after the second shower. A model with stronger BTU input will rebound faster.
Example 2: Couple, 1 bath + occasional laundry (tankless)
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Simultaneous: One shower (2.0 GPM) + quick kitchen draw (0.8–1.0 GPM) → ~3.0 GPM
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ΔT: Winter inlet 50°F, outlet 110°F → ΔT 60°F
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Target: Tankless delivering ≥3.0 GPM at 60°F rise (choose 3.5–4.0 GPM @ 60°F for wiggle room)
Example 3: Three showers can overlap (tankless)
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Simultaneous: Three showers × 2.0 GPM = 6.0 GPM
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ΔT: 65–70°F in winter inlets
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Target: 6.0–7.0 GPM @ 70°F rise (consider a high-capacity unit or dual tankless in parallel)
FAQs I get all the time
Q: Is bigger always better?
Not necessarily. Oversizing can cost more up front and, in some cases, reduce efficiency. For tanks, aim for FHR that matches your real peak hour (with modest buffer). For tankless, size to real simultaneous GPM at winter ΔT.
Q: What about electric vs. gas?
Electric storage often has slower recovery (kW vs. BTU of gas). That might mean choosing a larger tank (higher FHR) to compensate—especially for back-to-back showers.
Q: How do I handle hard water?
Hard water shortens life and performance, especially for tankless systems (scaling). Budget for annual descaling or a conditioner.
Q: Can I just copy my neighbor’s size?
Your family’s peaks, inlet temps, and fixtures are unique. Take the 20 minutes to run the worksheet—it’s so worth it.
Where to browse models
When you’re ready, explore the State lineup (tank and tankless) and filter by FHR/GPM, BTU input, and warranty to build your shortlist. If you want, send me your worksheet numbers—I’ll sanity-check your picks in minutes.
(And yes, I kept the phrase “state water heaters” to a single, tidy cameo—right here. ✅)
Why this method works (and why it’s different from guesswork)
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It anchors your choice to peak-hour reality (not daily averages).
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It forces a check on ΔT (winter counts!) and recovery/BTU (for tanks) or GPM @ ΔT (for tankless).
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It bakes in your household patterns—stacked showers, tub nights, and those multi-tasking kitchen moments.
If you’d like, I can turn this into a printable one-pager with your numbers filled in—or build a small calculator to do the math for you.
In the next blog, you will know more about "Electric vs. Gas State Water Heaters: What’s the Better Fit for You?".
