7 free construction tools for UK subcontractors

You don't always need a full project system to answer a single commercial question. Sometimes you need to check what 5% retention looks like on a contract, whether a selling price leaves enough margin, or how a CIS deduction changes a payment. That's where BuildQS's free construction tools fit.

The seven tools below cover those checks, a simple payment application summary and the cost of employing someone. Their basic calculations are available without signing in. Optional AI analysis and account-linked features are separate and can require an account or eligible access.

Here's which one to open, what figures to have ready and where you still need to use your own judgement. All worked figures are illustrative examples, not customer results. Tool access and assumptions were checked on 21 October 2026.

Which free construction tool do you need?

Start with the question you're trying to answer:

• What does retention represent on this contract value? Use the retention calculator.
• I've got a retention balance. How do I split it into planning dates? Use the retention release schedule generator.
• I need a basic application summary. Open the payment application template generator.
• Is this selling price leaving the margin I expect? Try the project profitability calculator.
• What does this employee cost per productive hour? Use the true cost of an employee calculator.
• What does a selected CIS deduction rate do to this payment? Open the CIS deduction calculator.
• I'm reviewing the figures behind a late-payment interest estimate. See the late payment interest calculator, with the cautions below.

These are separate jobs. A retention calculation doesn't establish when your contract requires release, and a profit calculation doesn't tell you when the cash will arrive.

1. Retention calculator: put a value against the percentage

The retention calculator takes a contract value, retention percentage and practical completion date. It calculates retention on the value you enter and shows an illustrative split using dates six and twelve months after completion.

For example, enter a £120,000 contract value and a chosen 5% retention rate:

• Retention on the full entered value: £120,000 × 5% = £6,000.
• Half of that retention: £3,000.
• Remaining half: £3,000.

That gives you a quick view of the amount represented by the percentage. It doesn't mean £6,000 has already been deducted from applications on a partly completed job. To establish the balance actually held, reconcile your applications, certificates and any releases.

The date display needs the same care. This calculator's six- and twelve-month offsets are a model, not a reading of your subcontract. If your terms release money at practical completion, use those terms rather than adopting the displayed dates.

If you want more background before entering figures, read our guide to retention in construction.

2. Retention release schedule generator: start with money already held

Use the retention release schedule generator when you already know the retention balance and want to arrange it into planning dates. Unlike the first calculator, its starting point is the retention amount itself, not the contract value.

Have three things ready: the retention currently held, the practical completion date and the release arrangement you're modelling. The generator offers a six-plus-six option, a single twelve-month release and custom month offsets.

Suppose your reconciled retention balance is £6,000 and practical completion is 15 October 2026. Under the tool's six-plus-six option, the model shows:

• £3,000 on 15 April 2027, six months after that date.
• £3,000 on 15 October 2027, twelve months after it.

The custom option changes the month offsets; it still uses an equal two-part split. Don't assume it can represent every bespoke percentage arrangement.

Before adding either date to a cash plan, check the contract's release conditions and the evidence needed. A modelled release date is not a promise that money will reach your bank that day.

3. Payment application template generator: prepare a simple summary

The payment application template generator brings project details and a small set of valuation figures into one summary. Enter the subcontractor, client, project, application number and valuation date, then add this application's gross value, retention percentage and previous certified value.

The important input is this application's gross value, not the cumulative value of all work completed.

Using a fictional application with £18,000 of work this period, 5% retention and £42,000 previously certified, the tool calculates:

• Retention this application: £18,000 × 5% = £900.
• Net this application after that retention: £17,100.
• Cumulative applied value in the summary: £42,000 + £18,000 = £60,000.

Notice what happens to the previous certified value. The tool adds it to the current gross value for the cumulative summary; it does not subtract £42,000 from this month's £18,000.

This is a simple summary, not a complete valuation of your works. It doesn't decide what has been completed or approved. Keep your measured breakdown, variation evidence and any other deductions alongside it, and check whether the payer requires a particular format. Its net figure is not a full VAT, CIS or payment-certificate calculation.

Our payment application checklist covers the supporting information to consider before submission.

4. Project profitability calculator: check margin, not just the uplift

The project profitability calculator has an existing-margin check for when you know the selling price and project costs. It also has a desired-margin planning mode. For a straightforward review, start with the existing-margin calculation and be clear about which costs you've included.

Take a selling price of £100,000 excluding VAT and entered project costs of £80,000:

• Profit against those entered costs is £20,000.
• Margin is profit divided by selling price: £20,000 ÷ £100,000 = 20%.
• Markup is profit divided by cost: £20,000 ÷ £80,000 = 25%.

Those percentages describe the same figures using different bases. Adding 20% to an £80,000 cost gives a £96,000 price, not a 20% margin. For that margin, the price would be £80,000 ÷ 0.8 = £100,000.

The calculation is only as complete as the costs you enter. Don't read a result based on labour and materials alone as profit after all business overheads. Use the tool's relevant planning inputs, or keep a separate allowance for costs outside your chosen calculation.

For a fuller explanation of the distinction, see construction profit margins and the calculator.

5. Employee true cost calculator: look beyond the hourly pay rate

The employee true cost calculator estimates employer cost per productive hour from pay, working hours, non-labour time, holiday and employer contribution assumptions. You'll also see it labelled the True Labour Cost Calculator on the page.

An employee paid £20 an hour for 40 hours has £800 of basic weekly pay. But if you allocate five of those paid hours to non-labour time, there are 35 productive hours in that weekly assumption. Even before employer contributions and holiday are considered, £800 divided by 35 is about £22.86 per productive hour.

That small calculation explains why using the pay rate alone can understate the labour cost you're allowing for. It is not the tool's full employer-cost result.

Enter the employee's pay and hours, then review the pension setting, holiday and advanced assumptions before using the output in an estimate. At the time of review, the calculator's starting assumptions were based on 2025/26 UK settings. Don't assume they are the correct settings for a later tax year or your employee's circumstances.

Treat it as an estimating aid, not a replacement for your payroll records or advice from your accountant. Allow separately for other costs your estimate needs to cover, such as a van, equipment or supervision.

6. CIS deduction calculator: see the effect of the selected rate

The CIS deduction calculator takes a gross payment, materials cost and selected deduction rate. It shows the resulting deduction and net payment. You can select 20%, 30% or gross payment status at 0%; selecting an option does not verify the subcontractor's status with HMRC.

For a simplified example, assume £10,000 excluding VAT, £3,000 of eligible materials at actual cost and a 20% deduction rate confirmed through HMRC verification. Assume there are no other allowable exclusions or adjustments:

• Amount left after the entered materials: £10,000 − £3,000 = £7,000.
• CIS deduction: £7,000 × 20% = £1,400.
• Payment after that deduction: £10,000 − £1,400 = £8,600.

HMRC's guidance explains what to exclude before making a deduction, including VAT and qualifying costs. The calculator's two amount fields don't decide which costs qualify or handle every possible invoice adjustment. Check the supporting records and HMRC's deduction instructions rather than treating the materials field as a general deductions box.

Keep retention and CIS separate in your reconciliation. They are different deductions, and the standalone CIS tool is not a combined payment application calculation.

7. Late payment interest calculator: check the assumptions first

The late payment interest calculator has fields for the payment amount and dates. Before relying on any displayed estimate, establish the relevant payment date, check the interest provisions in the contract and confirm which rules apply to the debt.

An overdue amount on your ledger doesn't, by itself, tell the calculator whether a particular interest regime applies. Nor can entering a date resolve a dispute about the correct contractual date.

GOV.UK's guidance on commercial late-payment interest explains that contract interest provisions affect the position. Check that guidance and obtain contract-specific advice where needed. A displayed calculator figure is not confirmation of legal entitlement or of an amount you can recover.

Keep a record of the inputs, not just the answer

Whichever tool you use, keep the assumptions beside the result. A figure copied into an estimate without its cost basis or retention terms is difficult to check later.

Before using a calculation in your work:

1. Match the inputs to the right contract, valuation period or payment.
2. Confirm whether each figure is gross, net, cumulative or this period only.
3. Check the rate, date and tax-year assumptions that apply.
4. Reconcile the result with your project records before issuing anything.

The free tools suit a standalone check. For ongoing work, BuildQS also provides payment application calculations and retention tracking within the main product. Keep that distinction in mind: a public calculator result is not a saved project record or a record of what the payer has certified.

Frequently asked questions

Are the BuildQS construction tools free without signing up?

The basic calculations in these seven tools were accessible without signing in when checked on 21 October 2026. Optional AI analysis and account-linked features can require an account or eligible access. You don't need those extras to follow the worked examples above.

Which retention tool should I use?

Use the retention calculator to calculate retention on an entered contract value. Use the release schedule generator when you already have a retention balance and want to model its split and dates. Neither replaces the release provisions in your subcontract.

Is the free payment application template a complete valuation?

No. It produces a simple summary from the figures you enter. You still need to check the valuation breakdown, evidence, contractual format and deductions that aren't covered by the tool. Enter this period's gross value, not the cumulative valuation.

Can I use the calculators instead of tax or legal advice?

No. They are calculation and planning aids. CIS treatment depends on the relevant HMRC rules and verified status, labour-cost assumptions need checking, and retention or interest questions can depend on your contract. Get professional advice where the figures or interpretation are uncertain.

Prefer to walk through your workflow live first? Book a Demo

Sources

• BuildQS free tools, public interactive pages and calculation behaviour checked on 21 October 2026. Direct links to each tool appear above.
• HMRC: Make deductions and pay subcontractors, accessed 21 October 2026. Guidance on verification-led rates and exclusions before calculating CIS deductions.
• GOV.UK: Interest on late commercial payments, accessed 21 October 2026. Guidance on the role of contract interest provisions.

All contract values, costs, dates and worked calculations in this article are illustrative. They are not market statistics, customer outcomes or a statement of your contractual entitlement.