Water treatment calculators

The numbers behind every glass of safe water.

36 calculators and models for plant operators and designers, aligned to NZ and global standards. Free to start. Nothing to install. Works on shift, on a phone.

Browse calculatorsMeet Opmate
TeamsPlans for teams of 3 or more, with invoice billing for NZ organisations
36tools
and counting
Free to useBasic tabs on every tool, forever
DWQAR alignedBuilt for the 2027 Rules
30 yearsof plant experience behind the design
The index

Every tool, by discipline.

DWQAR Sample SchedulerNZSmart sample schedule generator for all supply types for new 2027 DWQAR rules.
Filter Monitoring ToolMonitor long term performance of filters. Backwash profiles, media depths and dirt content.
Jar Testing ToolUPDATEDSet up mixing, make solutions, determine dosing regimen and assess results.
Operator Maths ToolAll the maths functions and calcs that operators need to know and use.
Frontline Operations Staffing ToolUPDATEDWork out how many frontline staff you need, or assess risk of your current levels.
Plant Audit ModelStructured audit framework to assess and benchmark plants.
Plant Capacity PlannerNEWPredicts treated water capacity through raw water quality events, hour by hour, showing plant bottlenecks in residuals handling processes.
Compliance Indices ToolNZDetermine compliance and assurance indices for suppliers, supplies, plants and zones.
Dechlorination ToolDetermine dechlorination requirements and impacts for tanks and pipelines.
PID Tuning ToolTune water treatment control loops - flow, level, pressure and feedback dose control.
Critical Control Point ToolNZDevelop your CCP register and responses based on DWQAR and best practice.
DWQAR Assurance Rule ToolNZDevelop your assurance and reporting rules strategy and response.
Carbon Footprint ToolNZCreate an operational carbon inventory for a drinking water supply, from the source to the tap.
Lake & Reservoir Management ToolCharacterise water bodies, track and identify issues and assess mitigations.
Clarifier Sludge Testing ToolConduct and assess results of sludge blanket settlement tests.
Treatment Process SelectorNZScreen treatment process trains against raw water quality and DWQAR.
Multi-Criteria Analysis ToolUPDATEDMake transparent decisions. Set up, score and assess decision sensitivity.
Plant Mass Balance ToolFull plant mass and flow balance for a range of scenarios, including residuals management.
Filter Design ToolSize rapid gravity filters including media, backwash & internals.
MF/UF Membrane Design ToolUPDATEDNZSize and manage MF/UF membrane plants including configuration, LRV and repairs.
UV Design ToolNZSize and model UV plants for disinfection and advanced oxidation.
Chlorination Design ToolSize and model chlorine gas and hypo storage, dosing and contact tanks.
Cost Estimating ToolCreate capex, opex and NPVs for multiple options, with multiple contingency models.
New Source Sampling ToolNZDetermine the optimum sampling regime for any new source type.
Clarifier Design ToolSelect and size clarifiers including flocculation and internals.
Future Demand Planning ToolNEWDetermine when system demand exceeds source, treatment and storage capacity.
Dosing System Design ToolNEWDesign liquid chemical dosing systems from storage to dose point.
Chlorine Demand & DBP ModelModel chlorine residual decay and disinfection by-product formation.
Water Chemistry ModelUPDATEDpH, alkalinity, LSI, CCPP, corrosion indices, plant sizing and dosing costs.
Arsenic ToolNZDetermine arsenic type and assess treatment options for removing.
Nitrate ToolNZDetermine nitrate/nitrite risk and assess treatment options for removing.
Oxidation ToolDetermine the optimum process for oxidation of Fe, Mn & ammonia.
PAC Dosing ToolDetermine PAC dose required for removal of organic contaminants.
Plant Hydraulic ModelUPDATEDCalculate the hydraulic grade line for gravity flow plants for a range of flowrates.
Pump System CalculatorUPDATEDPump curves, system curves, VSD modelling, costs and surge analysis.
Wet Well & Tank SizerSize wet wells, washwater and FTW tanks for continuous and batch operations.
Estimates, not professional advice

Every tool produces estimates based on real operations and design experience. However, the outputs do not constitute engineering, regulatory, scientific, legal, or other professional advice, and are not a substitute for the judgement of a suitably qualified and experienced professional who has assessed your specific circumstances.

Missing a tool, or spotted something wrong? Tell us.
Featured tool · Plant Audit

Every plant, scored the same way.

Inherited a set of plants you've never seen? Give every site audit the same structure and get one weighted score per plant, with a clear view of where it's strong and where it isn't.

Open Plant Audit
Built around DWQAR, water safety plans and CCPs.
OPMATE · LAUNCHING SOON

Calculators give you numbers. Opmate gives you answers.

Built on 30 years of operations troubleshooting experience. Opmate diagnoses plant issues, runs the calculators for you, and reads your Ops Manual, WSP, SOPs and SWRMP.

Join the waitlist
Opmate · live
Headloss has climbed steeply on all filters. Filter turbidity is low, source is stable and the clarifier looks good. What's going on?
When the plant is stable, filter turbidity is low and headloss behaves like that, it is almost always overdosing of poly. A quick visual check for poly overdosing is to see if the water surface of the clarifier and/or filters looks shiny and viscous.

Steps we need to take:

  1. Check the poly dose setpoint. For stable conditions it should be around 0.05mg/L.
  2. Do a dose time to confirm if the dose pump is delivering this.
  3. Check the batching system. We may have made up a batch that is too strong.
Why Premium

Replace the spreadsheets, rules of thumb and outsourced calcs.

Across all 36 toolsFreePremium
Basic calculationsEvery tool
IncludedIncluded
Simple scenariosRun the common cases
IncludedIncluded
Complex scenariosMultiple plants, edge cases, what-ifs
Not includedIncluded
Advanced analysis & diagnostic insightsWhy the numbers look the way they do
Not includedIncluded
Compliance checksAgainst DWQAR 2026 and DWSNZ 2022. Many are free; Premium has the full set
SomeIncluded
Save & share scenariosReview, reuse and compare across plants
Not includedIncluded
PDF & CSV reportsReady for management or the regulator
Not includedIncluded

What it means for a team responsible for several supplies

01Consistent, defensible answersBuilt by a specialist with 30 years of plant experience and aligned to DWQAR 2026 and DWSNZ 2022.
02Work done once, used everywhereSaved scenarios for a plant or a whole supply can be reviewed, reused and compared across the network, instead of rebuilt each time.
03Less key-person riskPlanning tools are built for whole supplies, not just plants. Shared, saved work keeps that knowledge with the organisation, not in one person's hands.
04A clear recordWhen a decision has to be justified to management or the regulator, the inputs, results and reasoning are there.
For a fraction of the cost of a single consultant engagement.An always-available technical toolkit to prepare for the 2027 compliance changes, troubleshoot faster, and make better operational and investment decisions.
See pricingPrice your team

Free to use.
Premium to unlock more.

For one person
Most popular

Premium

NZ$49/month

Access to advanced calculator features.

  • Advanced calculator functions
  • Save & import
  • PDF & CSV export
For teams of 3 or more

Team

NZ$45/seat/month

Less per seat as your team grows. One admin manages access.

5 seatsNZ$225/mo$45 a seat
10 seatsNZ$405/mo$40.50 a seat
25 seatsNZ$910/mo$36.40 a seat
  • Everything in Premium, for everyone
  • Pay by card, or invoice on 30-day terms (NZ)
  • Save 10% by paying annually
Work out your team price

Applicable taxes added at checkout.

Dr Jason Colton, founder of Watercalcs
Who built this

“All my learnings have gone into the design and testing of these tools.”

Dr Jason Colton — water treatment specialist. 30 years across operations, contracting and consulting in the UK, Asia, the USA, Australia and New Zealand. Doctorate in water process engineering, specialising in filtration.

FAQ

What is Watercalcs?

Watercalcs is a suite of online drinking water treatment calculators for plant operators, engineers and councils. It covers chemical dosing, disinfection, filtration, hydraulics, process design, cost estimating and New Zealand regulatory compliance. Everything runs in a browser with nothing to install.

Who is Watercalcs for?

Water treatment plant operators, process and design engineers, council and council-controlled organisation staff, and consultants working on drinking water supplies. It is built around New Zealand and Australian practice, though most of the engineering tools apply anywhere.

Does Watercalcs cover Taumata Arowai and DWQAR requirements?

Yes. Several tools are built specifically around the New Zealand Water Services Drinking Water Quality Assurance Rules, including a DWQAR sample scheduler, an assurance rule tool, a critical control point register builder and compliance indices. These reflect the 2026 Rules, which come into force on 1 July 2027.

What calculators are included?

Water chemistry and corrosion indices, chlorine demand and disinfection by-products, chlorination and UV design, filtration and membrane design, jar testing and coagulation, pump and hydraulic modelling, tank sizing, mass balance, plant auditing, operator staffing, cost estimating, carbon footprint, and a set of New Zealand compliance tools.

How much does Watercalcs cost?

Every calculator has free basic tabs with no registration required. Premium unlocks the advanced tabs for an individual subscription, and team plans start at three seats with per-seat pricing that reduces as the team grows. All prices exclude applicable taxes.

Can my council or organisation buy Watercalcs for a team?

Yes. Team plans cover three or more people with an administrator who manages who has access. New Zealand organisations can pay by invoice on 30-day terms with a purchase order number, which suits council procurement. Everyone else pays by card or direct debit.

Do I need to install anything?

No. Every calculator runs in a web browser and works on locked-down plant machines without software installation or administrator rights. The tools are also designed to work on a phone, on shift.

Are the calculations reliable?

Each tool states its assumptions and method, and is checked against real plant data before release. The outputs are engineering estimates for comparison and preliminary design. They are not professional advice, and you should verify results against jar testing, plant trials, laboratory analysis and the applicable standards before acting on them.

Who built Watercalcs?

Watercalcs is built by Dr Jason Colton, a water treatment specialist with a 30-year career across operations, contracting and consulting in the UK, Asia, the USA, Australia and New Zealand, and a doctorate in water process engineering specialising in filtration. It is operated by Water Software Solutions Ltd, a New Zealand company.