Calculators by profession

Ohm's law, cable voltage drop and watts to amps: quick electrical sums for sizing circuits and checking loads.

• Cable Voltage Drop Calculator

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  Check the voltage drop on a copper cable run against the UK wiring regulations' 3% and 5% limits, with the longest run and smallest cable that pass.

• Ohm's Law Calculator

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  Find voltage, current or resistance from the other two using Ohm's law, or a heater element's resistance from its watts. Shows the power.

• Watts to Amps Calculator

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  Convert watts to amps, or amps to watts, for DC, single-phase or three-phase AC, with power factor. Shows the working.

Materials and measurements for building and decorating: concrete, bricks, gravel, flooring, tiles, paint, wallpaper and roofs.

• Concrete Calculator

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  Work out how much concrete you need for a slab, base or path, with extra for waste and an approximate weight.

• Gravel Calculator

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  Work out how many tonnes of gravel, MOT Type 1 or sand you need for a driveway, path or base, and how many bulk bags or small bags that is.

• Tile Calculator

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  Work out how many tiles and boxes you need for a floor or wall, allowing for grout gaps and extra for cuts and breakages.

• Paint Calculator

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  Work out how much paint you need for a room's walls or ceiling, allowing for doors, windows and the number of coats, with tin sizes to buy.

• Wallpaper Calculator

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  Work out how many rolls of wallpaper you need for a room or feature wall, allowing for the pattern repeat.

• Roof Pitch Calculator

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  Find a roof's pitch from its rise and run, or the roof height from the pitch. In degrees, rise in 12 and percent, with the rafter length.

Workshop sums: speeds and feeds, tap drill sizes, bolt torque, bolt circles, gear ratios and beam deflection.

• Tap Drill Size Calculator

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  Find the right tap drill size for metric, UNC and UNF threads, at any thread percentage, with the nearest standard drill and the thread it gives.

• Bolt Torque Calculator

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  Work out the tightening torque and clamp force for metric and UNC/UNF bolts by grade and lubrication, from 4.6 to 12.9 and SAE Grade 2, 5 and 8.

• Bolt Circle Calculator (PCD)

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  Find the X and Y position of every hole on a bolt circle or pitch circle diameter (PCD), plus the angle and distance between neighbouring holes.

• Screw Thread Turns Calculator

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  How many turns of a screw or thread to move a set distance, or how far a number of turns moves it: metric, UNC, UNF or any pitch, in turns and degrees.

• Gear Ratio Calculator

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  Work out a gear ratio from the number of teeth, and the output speed and torque from the input, allowing for efficiency.

• Beam Deflection Calculator

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  Work out how much a simply supported or cantilever beam bends under an even or point load, with the bending moment, for timber, steel and more.

For sole traders and freelancers: VAT, tax, day rates, margins, break-even and late payment interest.

• VAT Calculator (UK)

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  Add VAT to a price or take it off, at 20%, 5% or any rate. Shows the VAT amount and the prices with and without VAT.

• Sole Trader Tax Calculator (UK)

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  Work out the income tax and Class 4 National Insurance you'll pay as a self-employed sole trader, and how much to set aside each month.

• Freelance Day Rate Calculator

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  Work out the day rate and hourly rate you need to charge as a freelancer or contractor, allowing for holidays, costs and unpaid admin time.

• Profit Margin and Markup Calculator

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  Work out profit margin and markup from cost and price, or the selling price you need for a target margin or markup.

• Late Payment Interest Calculator (UK)

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  Work out the statutory interest and compensation you can claim on a late business invoice in the UK: 8% plus the Bank of England base rate.