Engineering tools

Free engineering tools

Calculators and utilities we use in our own engineering work, published free. Every tool shows its formula, assumptions, a worked example and its limitations, so you can check the result by hand.

These tools run entirely in your browser; nothing you enter is sent to us. They are engineering aids, not a substitute for the applicable standard or a qualified engineer's sign-off.

Electrical

Calculators for cable sizing and short-circuit withstand, voltage drop, wire gauges, power and current, power factor correction, motor starters, transformer sizing and fault current, earth electrodes and lighting by the lumen method, with the formulas and limits behind them.

Voltage drop calculator

Calculate voltage drop and percentage drop for DC, single-phase and three-phase circuits from cable length, current, conductor size and material, and find the smallest conductor that meets your limit.

Cable sizing calculator

Choose a copper conductor size to IEC 60364-5-52 from the design current and breaker rating, the installation method, PVC or XLPE insulation, ambient temperature and grouping, with the Ib ≤ In ≤ Iz coordination and the voltage drop over the route checked together.

Cable short-circuit withstand calculator

Check a conductor against the adiabatic equation of IEC 60364-4-43: the fault I²t, the smallest size that survives it, what your chosen size can absorb, the longest clearing time and the largest current it tolerates, for copper or aluminium, PVC or XLPE, line or protective conductors.

Wire gauge converter (AWG, mm², diameter)

Convert an AWG number, a cross-section in mm² or a measured diameter into the other two, find the nearest and next-up IEC 60228 size, and get the resistance per kilometre at any temperature, the resistance of a run and its loop, and the mass of copper or aluminium. With an SWG and 7/029 table.

Power and current calculator

Convert between kW, kVA and amperes for DC, single-phase and three-phase circuits, with power factor and motor efficiency taken into account, and see the reactive power and phase angle that go with them.

Motor starter sizing calculator

Find a motor's full-load current from its nameplate data, compare the starting current and torque of direct-on-line, star-delta, soft starter and drive, then pick the AC-3 contactor, the overload relay setting, the short-circuit device and the cable design current for the method you choose.

Power factor correction calculator

Work out the capacitor bank (kVAr) needed to raise a load's power factor to a target, the capacitance per phase for delta or star connection, and how much the apparent power and line current fall as a result.

Transformer sizing and fault current calculator

Size a distribution transformer from the connected load, demand factor, power factor, growth and planned loading, pick the next IEC 60076 standard kVA, and get the full-load currents on both sides and the short-circuit current at the LV terminals from the percent impedance.

Lighting calculator (lumen method)

Work out how many luminaires a room needs for a target maintained illuminance with the lumen method: room index, utilisation and maintenance factors, a rectangular grid with its spacing-to-height ratio, the lux that grid achieves, and the lighting power density in W/m².

Earth electrode resistance calculator

Estimate the resistance to earth of a driven rod from the soil resistivity, its length and diameter, see what more rods in a line, a longer rod or better soil would do, find how many rods a target needs, and check the TT touch-voltage limit for the RCD you use.

Energy

Calculators for battery backup runtime and charging time, UPS sizing, solar array sizing, panel tilt, charge controllers and payback under net metering, generator sizing and fuel cost, and appliance running cost, with the assumptions each one makes spelled out.

Battery backup runtime calculator

Estimate how long a battery bank will run a load through a UPS or inverter, and the capacity needed for a target runtime, accounting for depth of discharge and inverter efficiency.

UPS sizing calculator

Size a UPS for an office, network rack or small server room: VA and watts with a margin to the next standard rating, the battery energy for the runtime through the inverter efficiency and the usable share of lead-acid at that rate, the bank in 12 V blocks, the C-rate and the recharge time.

Battery charging time calculator

Estimate how long a lead-acid or LiFePO4 battery takes to recharge from its present state of charge at a given charger current: the amp-hours to replace, the bulk and absorption stages, a check of the charge rate against the chemistry, and the current needed for an overnight refill.

Solar system sizing calculator

Size a solar PV array from your daily energy use and local peak sun hours, allowing for system losses; then the number of panels, the battery bank for the autonomy you want, and the inverter rating for your peak load.

Solar panel tilt calculator

Find the best fixed tilt for your latitude, the two-position seasonal tilts, and how much yield an array at your actual roof tilt and compass direction gives up against the optimum, from a clear-sky model integrated over the year.

Solar payback calculator

Build the year-by-year cash flow of a net-metered solar system from its size, cost, specific yield, self-used share, import tariff and export credit, with degradation, tariff escalation and an O&M allowance, and read off the simple and discounted payback, NPV, net benefit and LCOE.

Charge controller sizing calculator

Size an MPPT or PWM solar charge controller from the module datasheet, the string layout and the battery voltage: the string's Voc on the coldest morning, its Vmp on a hot afternoon, the array current with the 1.25 factor, the controller rating and PV voltage class, and what a PWM unit gives up.

Generator sizing calculator

Size a standby diesel or gas generator from the running load and the largest motor it must start, with the starting method, the set's overload capability, a growth margin and derating for altitude and ambient temperature.

Generator fuel consumption and running cost calculator

Work out what a diesel generator burns at the load it actually carries, in litres per hour and per kWh, and turn it into the cost per unit against the grid tariff, the litres and rupees per day and per month, and the CO₂, from the set rating and its datasheet consumption.

Appliance electricity cost calculator

Work out what an appliance costs to run: units per day, month and year from its rated power, the share of the time it actually draws that power, the hours it is used and your tariff, with standby draw and a check against the residential tariff slabs.

Electronics

Calculators for the small decisions in circuit design: LED series resistors, voltage dividers, RC filters, 555 timers, heat sinks and junction temperature, battery life for low-power devices and PCB trace width, with the reasoning and the data behind each one.

LED series resistor calculator

Find the series resistor for one LED or a string of LEDs from the supply voltage, forward voltage and current; pick the nearest E12 or E24 standard value, see the current it actually gives, and get the resistor power rating to use.

Voltage divider calculator

Find R1 for a wanted output from a chosen R2, pick the nearest E12, E24 or E96 value, and see what the divider really delivers: the unloaded and loaded output, the current it draws, the power in each resistor, its Thévenin impedance and the worst case with resistor tolerance.

RC low-pass filter calculator

Cutoff frequency and time constant from R and C, or the nearest standard R or C for a target cutoff; the attenuation and phase shift at the frequency you care about, the step response from 63 % to 99.9 %, and the −20 dB per decade roll-off in numbers.

555 timer calculator (astable and monostable)

Frequency, period, high and low times and duty cycle of a 555 astable from R1, R2 and C; the nearest E-series R1 and R2 for a target frequency and duty; the monostable pulse width from R and C; with the discharge-pin current and the checks that catch a circuit that will not time as the formula says.

Heat sink and junction temperature calculator

The heat sink thermal resistance a transistor, MOSFET, regulator or diode needs from its dissipation, the ambient in the enclosure and a design junction temperature; the junction, case and sink temperatures with the sink you have; the same device with no sink; and the power each can take.

Battery life calculator for low-power devices

Estimate how long a sensor node or tracker will run on its battery: the average current from a sleep floor, wake and radio bursts, converter loss and self-discharge; the life in months; which drain dominates; and what doubling the events or halving the sleep current would do.

PCB trace width calculator (IPC-2221)

Find the copper trace width a current needs for a chosen temperature rise on an external or internal layer, for 0.5 to 3 oz copper, with the IPC-2221 formula; then the trace's resistance, voltage drop and loss, and a check of a width you already have: what it carries and how hot it runs.

Automation

Calculators for the control panel and the plant floor: scaling 4–20 mA and 0–10 V signals into engineering units and PLC counts, sizing pump motors from flow and head, PID tuning from a step test, PT100 resistance and wiring errors, and tank volume from level.

4–20 mA signal scaling calculator

Convert a 4–20 mA, 0–20 mA or 0–10 V signal into percent of span, engineering units and a PLC raw count, and back again, with the NAMUR NE 43 fault bands and the voltage the loop current produces across the receiver's sense resistor.

Pump power and motor sizing calculator

Work out hydraulic and shaft power from flow, head and fluid density, pick the next standard IEC motor rating with a margin, and see the electrical input, line current, motor loading and the specific energy in kWh per cubic metre that decides the electricity bill.

PID tuning calculator (step test and ultimate gain)

Turn an open-loop step test (process gain, time constant, dead time) or a closed-loop ultimate test (Ku, Pu) into P, PI or PID settings by the Ziegler–Nichols and SIMC rules, in gain and times or proportional band and repeats per minute, with the parallel-form Ki and Kd.

PT100 and PT1000 resistance–temperature calculator

Convert a PT100 or PT1000 temperature to resistance and back with the IEC 60751 Callendar–Van Dusen equation, see the sensitivity and tolerance class at that temperature, and find out how much a 2-, 3- or 4-wire connection with real lead resistance adds to the reading.

Tank volume and level calculator

Turn a measured level into litres for a horizontal cylinder, a vertical cylinder or a rectangular tank: capacity, volume and mass at the level, the 4–20 mA the transmitter gives, the error a linear PLC scaling makes on a horizontal tank, and a strapping table to load into the PLC.

How we build these tools

Each calculator is a small, self-contained script with its inputs, units and formula defined in code and reviewed like any other engineering deliverable. When we correct or extend a tool, the page notes the date of the last review. If you spot an error, tell us; we would rather fix it than have someone size a cable wrongly.