splash! is a friendly planning tool for estimating how a small outdoor over-ground pool warms up, cools down overnight, and what it may cost to hold a target temperature.
You can type your own 3-day forecast manually at any time, or, when you are online, search any country / city / town worldwide to auto-fill the next three days plus an hourly 2-day profile from free public weather services.
The planner blends pool size, heater output, tariffs, passive cover use, current water temperature, and short-term weather into one place so you can compare likely outcomes before spending money or time.
Quick start
1. Set your pool geometry (radius or rectangular X × Y), heater, tariff, and cover assumptions.
1b. Use the target-temperature preset list for common use-cases (public swim, children, therapy, spa) or keep a custom target.
2. Use either a measured current pool temperature or the model estimate, then type the next 3 days manually or fetch them from a searched global location.
2b. Optionally set pool-in and pool-away dates plus your water rate to unlock a season cost summary.
3. Review the forecast, charts, maths checks, and exports.
This calculator models garden pool heating using energy balance:
heater input + solar gain − environmental heat loss.
Outputs include monthly maintenance demand, target reachability, short-range forecast behaviour, and annual electricity cost using location-aware climate data when available (with safe fallback baselines).
Chemistry bot
🤖 Rough guide — always test first.
Before chatting, confirm your garden pool geometry and latest chemistry readings are filled in correctly.
Ask about your current chemistry readings and dosing cards.
Complete the Turnstile check to start chatting.
Your chat session stays temporary and in-memory only.
📊 Key Metrics
Pool Volume
—
litres
Thermal Capacity
—
kJ / °C
Thermal Time Constant
—
hours (63 % equilibration)
Max Temp — Continuous
—
°C above ambient (reference month)
Max Temp — Selected Schedule
—
°C above ambient (reference month)
Time to Heat Pool
—
hours, ambient → target (reference month)
Heat-up Energy (reference month)
—
kWh of heater input, ambient → target
Annual Electricity Cost
—
£ / year (all 12 months)
Overnight Drop (reference month)
—
°C from target, no heating
Cover Saving
—
£ / year vs no cover
Desired-Range Days
—
days / year reaching min desired temp
Ideal Target @ Cost Target
—
°C, based on selected setup
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🌡️ 2-Day Pool Temperature Forecast
Forecast trend from current pool temperature using 48 hourly weather points when available.
🗓️ Temporary Pool Season Costs
Season Duration
—
days (selected dates)
Season Electricity
—
£ heating, whole season
Water Fill Cost
—
£ fills × pool volume
Total Season Cost
—
£ electricity + water
Cost per Day
—
£ / day (season average)
🧭 Planning answers people usually ask
Can I stay in my desired range?
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Will my budget hold?
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How much will it cool overnight?
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What does the cover change?
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🔮 Boost model & 3-day prediction
Temp After Top-Ups
—
°C after hot water + kettles
Hot Water Needed
—
L from tank to hit target now
Kettle Fills Needed
—
boils to hit target now
Forecast Day 3 End
—
°C after 72-hour forecast
3-Day Boost Eq.
—
£ peak-rate equivalent for top-ups + extra heater
3-Day Forecast Simulation
Day
Forecast Air
Cloud Cover
Extra Heater
Start Temp
End Temp
Solar Gain
Extra Cost
Forecast results use a stepped day/night energy-balance model, begin from the current pool temperature after optional hot-water and kettle boosts, and cap total heater runtime at 24 hours per day.
Model review in v1.6.6
Core daylight-solar and forecast-cap safeguards remain in place. v1.6.6 fixes PWA data persistence: chemistry readings (FC, TC, TA, pH, CYA, CH, BR) and chlorine-type selection are now saved to localStorage as you type, and selected location coordinates are persisted immediately on dropdown selection, so all settings survive app restarts. v1.6.5 adds dedicated splash-specific hero artwork and preview metadata so shared `/splash` links present the app visually without changing the main Jahosi homepage preview image. v1.6.4 adds a version-manifest-driven HTML refresh path so browser and PWA users are pushed onto the latest shell sooner when a new release lands. v1.6.3 keeps the existing weather model intact while clarifying that chemistry targets are standard residential guideline bands rather than strip-brand-specific normals, with tighter default CYA guidance and clearer hardness caveats. v1.6.2 hardens save/restore behavior for weather-location context and improves launch-time weather refresh resilience while keeping manual entry as fallback. v1.6.0 extends near-term simulation to 2 days, removes the coarse 7-day schedule chart, and uses hourly shortwave radiation when available so near-term solar heat gain follows observed forecast conditions more directly. Manual forecasts remain planning estimates, not guarantees.
Export Reports
Download a shareable summary or raw monthly data including pool statistics, budget, target range, and cover assumptions.
💾 Configuration
Settings are automatically saved in your browser between sessions. Use Save / Import to back up or transfer your configuration.
Exports are generated locally in your browser.
Math Integrity Checks
Running checks…
📅 Monthly Analysis — Maintaining Target Temperature
Month
Avg Air Temp
Solar Gain (avg W)
Heat Loss at Target (W)
Net Heat Needed (W)
Daily Energy to Maintain
Heating Available
Max Temp Continuous
Max Temp Selected Schedule
Target Reachable?
Daily Cost (selected schedule)
Monthly Cost (actual days)
📈 Visualisations
Monthly Cost to Maintain Target Temperature
Heating Curve — Ambient to Target (continuous heater, reference month)
Heat-Loss Breakdown at Target Temp (reference month)
Ideal Target Temperature vs Annual Cost Target (selected variables)
CDC Healthy Swimming: hot tubs are typically run warmer than pools, commonly in the high-30s °C range.
Pool-owner forums and community discussions commonly report uncovered overnight drops of about 3–6 °C for small outdoor pools in cool or breezy conditions, with bubble covers sharply reducing that loss.
These public-query and forum-derived heuristics are used for planning prompts and cover assumptions; always treat them as indicative rather than guaranteed outcomes.
Why buried/permanent pools are not modelled yet
splash! currently targets temporary or lightly installed outdoor playpools and uses a simplified, single-body water model with public climate inputs. Fully buried or permanent pools need extra site-specific physics the app does not yet collect reliably, including layered wall/base construction, insulation thickness and type, soil moisture and drainage, ground-temperature lag, and nearby building effects on wind shelter, sky-view radiation, and shading. Without those inputs, one-size-fits-all buried-pool outputs would look precise but often be wrong, so this version keeps buried/permanent pool modelling out of scope and treats results as planning estimates, not guarantees.
🧪 Pool Chemistry Guide (Chlorine Pools)
⚠ Rough guide — estimates only. Always test with test strips or a drop-test kit first.
Dosing figures here are indicative starting points derived from industry-standard formulas and published reference data
(WHO, PWTAG, ANSI/APSP, Taylor Technologies). Actual outcomes depend on your specific water source,
temperature, bather load, UV exposure, organic load, and local water chemistry.
These target bands are standard guideline ranges for a typical residential outdoor chlorine pool — they are not intended to mirror one specific dip-stick brand, and strip packaging may show slightly different "normal" colours or labels.
Never add any chemical to your pool without first testing the water.
Test before and after every chemical addition, and allow mixing time before re-testing.
These values are not a substitute for professional water analysis or the advice of a qualified pool technician.
Overdosing is hazardous — when in doubt, add less and retest.
⚗️ Chemistry Configuration
—
°C
Estimated current temperature: —
Both dichlor and trichlor are stabilised chlorines — they contain cyanuric acid (CYA / stabiliser)
and will raise CYA levels over time with every dose.
Dichlor is typically granular and quick-dissolving; good for regular dosing.
Trichlor is typically slow-dissolving tablets or sticks used in a floating feeder.
Monitor CYA regularly and carry out a partial water change if CYA exceeds 90 ppm.
Enter your current test results (all fields optional)
Fill in your most recent test-strip or drop-test readings to receive personalised rough dosing estimates.
Readings update instantly as you type. Leave fields blank to hide recommendations for that parameter.
*With CYA stabiliser, minimum FC = max(1.0, 0.075 × CYA) per CDC MAHC 2023 (FC:CYA ratio). E.g., at CYA 50 ppm, min FC = 3.75 ppm. Below minimum: inadequate disinfection. Above 5 ppm: eye irritation, bleaching. (CDC MAHC 2023; WHO Vol 2, 2006)
Hardness (strip total hardness / calcium hardness guide)
150
200–400
500
ppm as CaCO₃
Presented as a broad default because strips often show total hardness while many references discuss calcium hardness. Vinyl / above-ground pools often tolerate the lower end; plaster / concrete pools commonly run higher. Above 500: scale risk rises. (ANSI/APSP/ICC 11-2019; Taylor Technologies Professional Reference)
Cyanuric Acid (CYA / Stabiliser)
30
40–60
90
ppm
Protects FC from UV photolysis in outdoor pools. This app uses a conservative domestic default band; some strip brands and operator guides show a wider 30–100 style band. Above 90: chlorine becomes harder to manage effectively and water dilution is usually needed. (CDC MAHC 2023; PWTAG 2017)
Combined Chlorine (CC)
0
0–0.2
0.5
ppm
CC = TC − FC. Chloramines cause the classic "chlorine smell" and eye/respiratory irritation. CC ≥ 0.5 ppm indicates a need for breakpoint chlorination: shock at ~10 × CC (use non-stabilised chlorine for shock to avoid further CYA rise). (WHO Vol 2, 2006; CDC MAHC 2023)
These are standard reference targets for typical residential outdoor chlorine pools. Hot tubs, indoor pools, vinyl/liner pools, plaster/concrete pools, and high-bather-load facilities may need different targets. Strip packaging may differ slightly; always treat these as rough guides and test your own water regularly with calibrated equipment.
🌡️ Estimated Rates of Change by Temperature
These are rough indicative estimates only. Actual rates depend heavily on bather load, direct sunlight, organic load, and local water chemistry.
Current pool temperature (from main config): — — active band: —.
Water Temperature
Typical FC Loss / Day
Recommended Check Interval
Notes
CYA Accumulation with Stabilised Chlorines
Configure chlorine type and pool volume (above) to see a CYA accumulation estimate.
FC depletion rates based on:
CDC Healthy Swimming ·
WHO Recreational Water Guidelines Vol 2 (2006) ·
PWTAG Technical Guidance ·
Wojtowicz J.C. (2001) "Effect of temperature on chemical equilibria in swimming pools", Journal of the Swimming Pool and Spa Industry;
White G.C. (1999) Handbook of Chlorination and Alternative Disinfectants, 4th ed., Wiley.
Rates assume CYA-stabilised chlorine at 40–80 ppm, low-to-moderate bather load, outdoor daytime use.
Approximate routine usage based on your selected location climate profile, current temperature band, and pool volume.
These are rough guides only and assume typical residential usage.
Hard vs soft water can materially change pH/alkalinity/hardness demand.
Maintenance Product
Estimated Quantity
Cadence
Why this may be needed
Set your location and chemistry type to calculate rough product quantities.
Water chemistry profile guidance will appear here (soft/neutral/hard scenarios).
📥 Download Chemistry Schedule
Generate a personalised pool chemistry schedule based on your pool's volume and chlorine type.
The schedule includes dosing reference amounts for your specific pool, temperature-based checking intervals,
a month-by-month planner using your location's climate data, and CYA accumulation guidance.
Opens a print-ready page in a new tab. Use your browser's Print → Save as PDF to save a PDF copy.
⚠ All dosing values are rough estimates. Always test your pool water first. These are planning guides, not prescriptions.
