Introducing MEPCalc: MEP calculations traced to CIBSE, BS 7671 and BS EN standards

MEPCalc is a web platform of more than 40 calculators for building-services engineers. Each calculator names the CIBSE, BS 7671 or BS EN method it implements, and every run can produce a PDF report that shows the inputs, the method, the intermediate steps and the governing check.

We built it because of the folder every MEP engineer has. A decade of inherited spreadsheets. The volt drop tab someone set up in 2014. The heat loss workbook with a macro nobody dares touch. They work, probably, but when a checker asks where a factor came from, the answer is often a shrug. Engineers rebuild a colleague's workbook just to trust it, and the same calculation gets done three different ways across one office.

What MEPCalc is

MEPCalc runs the routine calculations of a UK building-services design office in a browser, against the standard each one belongs to, and records the working. It covers electrical, HVAC, ventilation, ductwork, refrigeration, heating, pipework, plumbing, acoustics and renewables.

Three things make it different from a spreadsheet or a free online calculator.

The method is named, not implied. A cable size calculation says which BS 7671 Appendix 4 table it read and which reference method it used. A space load says whether it ran the CIBSE Guide A admittance method or the ASHRAE radiant time series. The reference data behind the calculators carries its source and has been checked against the published documents. Some of that checking is public: the verification pages run fixed worked examples through the live engines and print the expected values, our results and the differences.

The report is built for the checker. Every run can produce a PDF with the inputs, the method, the intermediate steps and the governing check. A checker can follow it line by line without opening the tool.

Calculations live in projects. The design standard is set at project level, so changing it once carries through to every new sheet in that project.

What MEPCalc calculates

The calculators, grouped the way the navigation groups them, with the standard each one implements.

Electrical

Circuit Design sizes a cable, checks the adiabatic condition, selects the protective device and verifies earth fault loop impedance in one pass to BS 7671:2018+A3:2024. Voltage Drop uses the BS 7671 Appendix 4 mV/A/m tables; the voltage drop formula is written up in the knowledge base. Fault Level follows BS 7671 Appendix 14. Load Schedule applies the IET On-Site Guide Table A2 diversity factors, and DB Schedule builds the board from it. Motor Starter follows IEC 60947-4-1. UPS follows IEC 62040-3:2021. Generator sizes to ISO 8528 including step-load acceptance. Emergency Lighting follows BS 5266-1:2025 and BS EN 1838:2024. Lighting Design uses the lumen method to BS EN 12464-1:2021. EV Charging follows BS 7671 Section 722. Power Factor Correction sizes the capacitor bank for a target power factor.

The BS 7671 cable size calculation is the most checked calculator on the site: the cable size verification page runs fixed examples from the standard and the On-Site Guide through the engine and shows every delta.

HVAC and refrigeration

Space Load runs the CIBSE Guide A cyclic admittance method or the ASHRAE Fundamentals radiant time series. Fabric Heat Loss follows BS EN 12831-1:2017, including the thermal bridging allowance and the adjacency factors. Solar Gain uses CIBSE Guide A solar data for the chosen station and orientation. Refrigeration Loads runs the IOR Cold Store Code of Practice 2020 method or the ASHRAE Refrigeration Handbook method. Refrigerant Pipe Sizing follows the ASHRAE Refrigeration Handbook 2022. Condensate Drain follows Approved Document H and BS EN 12056-2. Machinery Room checks the EN 378-3 requirements for refrigeration plant rooms.

Ventilation and ductwork

Ventilation Design sets rates to CIBSE Guide B2, Approved Document F or BS EN 16798-1. Whole Dwelling applies Approved Document F 2026 Table 1.3. Natural Ventilation uses the BS EN 16798-7 flow methods. AHU Duty follows CIBSE Guide B3. Standalone Fan checks specific fan power against the Part L limits, or in ASHRAE mode reports the AMCA 208 fan energy index. Duct Sizing and Ductwork Pressure Loss follow CIBSE Guide B2 with the Guide C fitting catalogue. Duct Insulation follows BS 5422:2023. Grille sizes face velocity and free area to CIBSE Guide B2. Louvre follows CIBSE Guide C. Duct Noise predicts the room NR level to CIBSE Guide B4.

Heating, pipework and plumbing

Heat Source Sizing follows CIBSE Guide B1 and AM16. Underfloor Heating applies the BS EN 1264-2:2021 surface temperature limits. Expansion Vessel follows BS EN 12828 Annex D. Hydronic Pipe Sizing and Pipe Insulation follow CIBSE Guide C and BS 5422:2023. Cold Water Pipe Sizing follows CIBSE Guide G. Hot Water Storage follows CIBSE Guide G. Drainage follows BS EN 12056-2 and 12056-3. Legionella Risk Assessment follows HSG274 Part 2 and the L8 Approved Code of Practice.

Renewables

Solar Thermal follows the SAP 10.3 Appendix H monthly method. PV Sizing follows SAP 10.3 Appendix M and MCS MIS 3002, with the G98 and G99 connection thresholds; the PV verification page shows the checks.

What a MEPCalc report shows

A MEPCalc report is written for the person checking the design, not the person who ran it. It lists every input, names the method and edition, shows the intermediate values and states the governing check with its margin. Where a value came from a table, the report says which table. The report carries the project details and is stored against the project, so the audit trail survives staff changes.

Who MEPCalc is for

Consultants who need defensible numbers quickly. Contractors who need to check a design without rebuilding it. Any engineer who has become the unpaid maintainer of a spreadsheet library. If you have ever re-derived a colleague's workbook just to trust it, this is the tool we built for you.

Where MEPCalc is now

We are onboarding testers now, with a public launch to follow. The roadmap after launch is set by what testers ask for. If you want a place in the next intake, join the waitlist. New users start with Your First Calculation in the help centre.

Frequently asked questions

Which software is used for MEP calculations in the UK?

Most UK design offices use a mix: spreadsheets built in house, a desktop cable package such as Amtech or ElectricalOM for BS 7671 work, a load calculation program for space loads, and free online calculators for one-off checks. MEPCalc replaces that mix with one web platform of more than 40 calculators, each named to its CIBSE, BS 7671 or BS EN method, with a PDF report for every run.

How do you estimate MEP works?

MEP works are estimated by sizing each system first: cable sizes and protective devices for the electrical distribution, heat loss and cooling load for the plant, ventilation rates and duct sizes for the air systems, and pipe sizes and storage for the water services. MEPCalc produces those design figures and the reports behind them. It does not produce cost estimates.

Is MEPCalc a replacement for engineering judgement?

No. It does the arithmetic and the reference table lookups. The inputs, the method assumptions and the results still need review by a competent engineer. The reports show the working so that review can happen.

Which standards does MEPCalc cover?

CIBSE Guides A, B1, B2, B3, B4, C and G, BS 7671:2018+A3:2024 and the IET On-Site Guide, BS EN 12831-1, BS EN 16798-1 and 16798-7, BS EN 1264-2, BS EN 12828, BS EN 12056, BS 5422:2023, BS 5266-1:2025, EN 378, HSG274, SAP 10.3 and MCS MIS 3001 and 3002. The verification pages show worked examples run against several of them.

When can I use MEPCalc?

MEPCalc is in a closed tester phase. Join the waitlist and you will be in the queue for the next intake.