Electrics & Tech Calculators

What the electrics and tech calculators cover

This shelf collects the sums that come up around electricity and computing: the relationship between volts, amps, ohms and watts; what an appliance costs to run; how much voltage a cable run loses; what a resistor's colour bands say; how long a download takes; how a network address splits into subnets; and how strong a password actually is. Seven tools, each one showing the formula with your numbers substituted in rather than just printing an answer. Every calculator here is free.

They divide naturally into two halves. On the electrical side sit the Ohms Law Calculator, the Electricity Cost Calculator, the Voltage Drop Calculator and the Resistor Calculator. On the tech side sit the Bandwidth Calculator, the IP Subnet Calculator and the Password Generator.

Ohm's law: the four quantities behind everything electrical

Almost every electrical sum on this shelf is Ohm's law wearing different clothes. Voltage equals current times resistance, V = I × R, and power ties in through P = V × I, which rearranges to P = I² × R and P = V² ÷ R. Know any two of the four quantities and the other two follow. The Ohms Law Calculator takes whichever two you have, names the formula it is using, substitutes your numbers into it, and derives all four values, with the full twelve-formula Ohm's law wheel underneath for reference.

The practical value is direction: the tool tells you which rearrangement answers your question, which is where most people slip. Working out the current a 2,400W heater draws from a 230V supply is P ÷ V, not V ÷ R; working out what resistance that implies is V ÷ I. Getting the rearrangement wrong by habit is the single most common Ohm's law mistake, and seeing the working spelt out is the cure.

Reading resistors off the component itself

The Resistor Calculator handles the notation side of the same territory. Pick the colours on a 4- or 5-band resistor and it reads them into a resistance, a tolerance and the range the true value can sit in; type a value instead and it tells you which colours to look for. A third tab totals a list of resistors in series, where values simply add, or in parallel, where the reciprocals add and the total always comes out smaller than the smallest resistor in the set, a result that surprises people the first time they meet it.

What electricity actually costs to run

An electricity bill charges for energy, not power, and the unit of sale is the kilowatt-hour. The Electricity Cost Calculator runs the two-line sum behind every appliance cost: kWh = watts ÷ 1,000 × hours, then cost = kWh × your unit price, worked out by day, week, month and year, next to a table of typical household appliances at the same rate.

The insight the tool keeps surfacing is that wattage alone does not predict cost, because time of use matters just as much. A 3,000W kettle boiling for ten minutes a day and a 60W television left on for four hours can cost roughly the same across a year, despite one being fifty times more powerful on paper. Anything that heats water or air draws power at a very high rate while it runs; electronics barely sip by comparison but often run for hours. One honest caveat the page states plainly: the calculator prices the appliance's energy at your unit rate and does not include the standing charge, the fixed daily amount every bill adds regardless of use.

When the cable itself eats your volts

The Voltage Drop Calculator answers a question that matters most on long runs and low voltages: how much of the supply is lost to the cable's own resistance before it reaches the load. The standard formula is drop = 2 × current × resistivity × one-way length ÷ cross-sectional area, the 2 accounting for the return conductor, using the reference resistivities for copper and aluminium at 20°C. The result comes with a percentage and a verdict against the commonly cited planning figures of about 3% for a branch or lighting circuit and 5% overall, which the page is careful to describe as guidance from the NEC's informational notes and BS 7671, not a legal maximum.

The counterintuitive part is why 12V systems suffer so badly. A device pulls power, and current is power divided by voltage, so halving the voltage doubles the current for the same load, and drop scales directly with current. A 120W load draws about 0.5A at 230V but around 10A at 12V, which is why a camper-van or garden lighting run needs far thicker cable per watt than a mains circuit ever does.

Downloads, data caps and the megabit trap

The Bandwidth Calculator exists mostly to settle one recurring confusion: connection speeds are quoted in megabits per second, file sizes in megabytes, and a byte is 8 bits. A 100 Mbps line moves 12.5 megabytes a second, not 100, so a download that seems eight times slower than the advertised speed is usually a units mistake, not a fault. Once both sides of the sum are in the same unit, time is simply size divided by speed, and the tool shows that division in full. An optional toggle knocks a rough 10% off the usable speed as a stand-in for protocol overhead and shared capacity, an honest allowance rather than a measurement. Further tabs turn a sustained speed into a data total, and a monthly cap into the number of days a habit will last.

Splitting a network into subnets

The IP Subnet Calculator takes an IPv4 address and a prefix length, or a subnet mask, and returns the full readout: network address, broadcast address, usable host range and count. The teaching layer is the binary: a /26 means the first 26 bits of the address are network bits, the mask is 255.255.255.192, and the network address is the input ANDed with that mask, bit by bit, which the calculator draws out underneath. Usable hosts follow as 2 to the power of the host bits, minus 2 for the network and broadcast addresses, with the /31 point-to-point exception under RFC 3021 and the single-address /32 handled properly rather than glossed over.

Passwords measured honestly

The Password Generator builds a random password, or a memorable multi-word passphrase, from the browser's own cryptographically secure random number generator rather than the ordinary Math.random. Alongside the result it works the strength sum: entropy in bits = length × log₂(alphabet size), with a crack-time table giving that figure honest context. The passphrase tab makes the same maths argue for a different style of secret: several random common words can carry more entropy than a shorter jumble of symbols, while being far easier to remember.

How the tools fit together

A typical DIY electrical question chains three of them. Start with the Ohms Law Calculator to find the current a load draws, feed that current into the Voltage Drop Calculator to size the cable run, then use the Electricity Cost Calculator to see what running it will cost across a year. On the tech side, the Bandwidth Calculator and IP Subnet Calculator cover the home-network questions that actually come up, and the Password Generator covers the account securing it all. For unit changes along the way, watts to kilowatts or metres to feet, the Conversion Calculator on the science shelf handles the conversions, and anyone pricing a heavier electrical habit against a petrol one can set this shelf's running costs beside the Fuel Cost Calculator.

Frequently asked questions

Why is my download slower than my advertised broadband speed?

Two reasons stack up. First, the units: plans are sold in megabits a second, download bars count megabytes, and dividing by 8 usually explains most of the gap. Second, overhead: packet headers, other devices sharing the line and the far server's own limits all take a share. The Bandwidth Calculator shows both effects in its working.

Does a phone charger left plugged in cost much?

Very little. A charger drawing about 5W for eight hours uses 0.04 kWh, a fraction of a penny at typical rates. High-wattage, short-burst appliances like kettles, and low-wattage, long-hours ones like fridges, dominate real bills; the Electricity Cost Calculator lets you rank your own.

Is 3% voltage drop a legal limit?

No. The 3% and 5% figures are widely cited planning recommendations, from the NEC's informational notes in the US and BS 7671 guidance in the UK, not enforced maximums. The Voltage Drop Calculator presents its verdict as a sanity check against that guidance, not a certificate of compliance.

Is a long passphrase really stronger than a short complex password?

Usually, yes. Strength is measured in bits of entropy, and length multiplies while complexity only nudges the alphabet size inside a logarithm. The Password Generator shows the entropy sum for both styles so you can compare them directly rather than argue from habit.