Unit Conversion Tables: Every Electrical Quantity

1. Why unit conversion matters when reading schematics

A schematic is a compressed language. The same digit 5 on a net can mean 5 V on a supply rail, 5 kΩ on a resistor, or 5 nF on a decoupling capacitor — the prefix between the number and the unit is what carries the magnitude. Conversion tables exist to let you move between those magnitudes on demand, so you can confirm that a 4.7 kΩ resistor is the same part as a 4700 Ω one, or that a 100 nF capacitor matches the 0.1 μF value called out in a datasheet.

Getting a prefix wrong is the classic silent error in electronics: nothing crashes, but every dependent calculation shifts by a factor of ten, a hundred, or a thousand. The tables below cover the complete SI prefix set, the units behind every major electrical quantity, and the pairs that are most often confused. An interactive converter at the bottom does the arithmetic for you, and a worked circuit shows how voltage, current, resistance and power relate in real units. Use the on‑page index to jump to any quantity.

2. The SI system: base and derived units

The International System of Units (SI) defines seven base quantities — length (metre), mass (kilogram), time (second), electric current (ampere), temperature (kelvin), amount of substance (mole) and luminous intensity (candela). Almost everything in an electrical schematic is a derived unit built from these: the volt is watts per ampere, the ohm is volts per ampere, the watt is joules per second, the farad is coulombs per volt, the henry is volt‑seconds per ampere, and the coulomb is ampere‑seconds. Because the system is coherent, these line up exactly — 1 W = 1 V·A = 1 J/s — so a conversion is only correct if the base dimensions match on both sides.

Derived units matter because they let you verify a conversion by dimensional analysis. If you claim “1 Wh = 3600 J”, you can check it: 1 W = 1 J/s, one hour = 3600 s, so 1 Wh = 1 J/s × 3600 s = 3600 J. That cross‑check catches most mistakes before they reach a board. When a quantity on a schematic is unfamiliar, writing out its base dimensions (kg, m, s, A) is the fastest way to confirm a table entry — or to see that two apparently different units are actually the same.

3. SI prefixes & engineering notation

A prefix is a shorthand for a power of ten attached to a unit: kilo (k) means ×10³, milli (m) means ×10⁻³, so 1 kΩ = 1000 Ω and 1 mA = 0.001 A. The prefixes that actually appear on schematics cluster in groups of three — kilo/mega/giga on the large side, milli/micro/nano on the small — which matches the three‑significant‑figure values most components come in. This is engineering notation, where the exponent is always a multiple of three, and it is why a resistor is written 4.7 kΩ (not 4.7×10³ Ω) and a capacitor 100 nF (not 1×10⁻⁷ F).

The full set spans twenty‑four named prefixes, from quetto? — quecto (q, 10⁻³⁰) up to quetta (Q, 10³⁰), with the four extreme values (r, q, R, Q) added in the 2022 SI revision. You will meet the middle of that range daily; the extremes mostly live in metrology and physics. The reference table in section 19 lists every prefix with its symbol, multiplier, scientific and decimal forms, and a worked example — it is sortable and searchable, and each symbol can be copied.

4. How to read a prefixed value

Reading a prefixed value is a three‑step habit: take the number, read the prefix, read the unit. 4.7 kΩ is 4.7 × 10³ Ω = 4700 Ω. 3.3 V is just 3.3 volts (no prefix). 100 nF is 100 × 10⁻⁹ F = 1×10⁻⁷ F = 0.1 μF. Before any calculation it is usually safest to convert everything to the base unit (Ω, V, A, F), do the arithmetic in base units, then express the answer in whichever prefix keeps the number between roughly 0.1 and 1000.

5. Voltage, current & charge

Voltage (symbol V, unit the volt) is the potential difference that pushes current; on a schematic it appears as rail labels (+5 V, 3V3, VCC, GND), a battery or supply symbol, or a measured node. The practical prefixes are kV, V, mV and μV: mains is hundreds of volts, logic rails are single‑digit volts, sensor and audio signals are millivolts, and op‑amp offsets are microvolts. Because voltage is so central, almost every other quantity is quoted relative to it — power = V·I, energy = V·Q, impedance = V/I.

Current (symbol I, unit the ampere) is the flow of charge, with charge Q in coulombs (I = dQ/dt). Schematics and datasheets use kA, A, mA, μA and nA: motor and supply currents are amps or kiloamps, a microcontroller’s quiescent current is microamps, and a high‑impedance input may draw nanoamps. The most common mistake here is misreading a datasheet’s “I_Q = 250 μA” as 250 mA — a thousand‑fold error that shows up as a battery that drains far too fast. Always confirm the prefix on current specs, and remember 1 mA = 0.001 A = 1000 μA.

6. Resistance, conductance, resistivity & conductivity

Resistance (symbol R, unit the ohm, Ω) opposes current flow and is the most frequently prefixed quantity on a schematic: TΩ for insulation and ESD protection, MΩ for high‑impedance inputs, kΩ for the everyday bias and pull‑up resistors, Ω for current paths, and mΩ for shunts and ESR. Conductance (symbol G, unit the siemens, S) is simply the reciprocal, G = 1/R, so a 2 kΩ resistor has a conductance of 0.5 mS. You will meet siemens more in admittance and op‑amp feedback than in discrete part numbers.

Beneath the ohm sit two material properties. Resistivity ρ (ohm‑metre, Ω·m) describes how strongly a material resists current per unit geometry, and conductivity σ (siemens per metre, S/m) is its reciprocal, σ = 1/ρ. They relate to a physical resistor by R = ρ·L/A (length over cross‑section), which is why wire gauge matters: copper is roughly 1.68×10⁻⁸ Ω·m (about 5.8×10⁷ S/m). Watch the mΩ ESR quoted on modern capacitors and the MΩ insulation resistance quoted on cables — both are prefixes doing real work, and confusing mΩ with Ω overstates a loss by a factor of a thousand.

7. Power & energy (and the W vs Wh trap)

Power (symbol P, unit the watt, W) is the rate of energy transfer, P = V·I = I²R = V²/R, with prefixes MW, kW, W, mW and μW: a wall outlet is kilowatt‑class, a chip dissipates milliwatts, and a reference settles at microwatts. Energy (symbol E, unit the joule, J) is power integrated over time, E = P·t, and the joule is rarely big enough on its own — so we use the watt‑hour (Wh), where 1 Wh = 1 W × 1 h = 3600 J, and the kilowatt‑hour (kWh), where 1 kWh = 3.6×10⁶ J = 3.6 MJ, the unit your electricity bill is built on.

8. Capacitance (F, μF, nF, pF)

Capacitance (symbol C, unit the farad, F) measures how much charge a component stores per volt, C = Q/V. A full farad is enormous for a discrete part, so real capacitors are almost always specified in microfarads (μF), nanofarads (nF) and picofarads (pF), each a thousand times smaller than the last: 1 μF = 1000 nF = 1,000,000 pF. Bulk and filter caps are μF, decoupling caps are nF (the ubiquitous 100 nF), and trimmer, timing and RF caps are pF. The stored energy is ½CV², which is why a large cap at high voltage can hold a dangerous charge even after power is removed.

9. Inductance & reactance (H, mH, μH, nH)

Inductance (symbol L, unit the henry, H) opposes changes in current and stores energy in its magnetic field, E = ½LI². Like the farad, a bare henry is large, so inductors are specified in mH, μH and nH, each 1000 times smaller: 1 mH = 1000 μH = 1,000,000 nH. Power‑supply chokes are μH to mH, RF and filter inductors are nH to μH, and the energy formula is why an inductor’s stored energy — and the voltage spike when you open its circuit — scales with the square of the current.

Inductance shows up in circuits through reactance, X, measured in ohms and frequency‑dependent: inductive reactance X_L = 2πfL = ωL, and capacitive reactance X_C = 1/(2πfC) = 1/(ωC). Together with resistance they form the impedance Z (ohms), |Z| = √(R²+X²), and its reciprocal the admittance Y (siemens), Y = 1/Z. Filter and matching design lives entirely in these units: a 10 μH inductor at 1 MHz has X_L ≈ 62.8 Ω, and a 100 nF cap at 1 MHz has X_C ≈ 1.59 Ω. Confusing μH with mH (a factor of 1000) silently moves every cutoff frequency by the same factor.

10. Frequency, angular frequency, period & wavelength

Frequency (symbol f, unit the hertz, Hz) counts cycles per second, and is prefixed kHz, MHz and GHz: audio is kHz, microcontrollers run at MHz, and Wi‑Fi and RF at GHz. Its twin is the period T (seconds), the time for one cycle, T = 1/f — so a 1 kHz signal has a 1 ms period. Wavelength λ (metres) ties frequency to the speed of light: λ = c/f, with c ≈ 3×10⁸ m/s, so a 100 MHz signal has a wavelength of 3 m. These three (f, T, λ) are the backbone of timing and RF work.

11. The power triangle (W, VA, var, power factor)

In AC and switched circuits, power splits into three related quantities. Real (active) power P is in watts (W) and is the power actually converted to heat, light or motion. Apparent power S, in volt‑amperes (VA), is the product of RMS voltage and RMS current, |S| = V_rms·I_rms. Reactive power Q, in volt‑amperes reactive (var), is the power that shuttles back and forth with inductors and capacitors. The three form a right triangle with |S|² = P² + Q², and the ratio pf = P/S = cos φ is the power factor, quoted as a number from 0 to 1 or as a percentage.

12. Magnetic quantities (flux, flux density, field strength)

Magnetic flux Φ (unit the weber, Wb, with 1 Wb = 1 V·s) is the total magnetic field passing through an area; flux density B (unit the tesla, T, with 1 T = 1 Wb/m²) is the flux per unit area. Both are heavily prefixed: transformer and motor flux is mWb to μWb, and field strength is mT to μT. For scale, a strong neodymium magnet is about 1.0–1.4 T at its surface, a refrigerator magnet a few mT, and the Earth’s field roughly 25–65 μT. Inductance links directly to flux: L = NΦ/I, and for a core, L ≈ N²·μ·A/l.

Magnetic field strength H is measured in ampere per metre (A/m) and is the “cause” (mmf per unit length), while B is the “effect” (flux density); they relate through permeability, B = μH, where μ is the material’s permeability. This is why a ferrite’s datasheet quotes μ (relative permeability, dimensionless) rather than a fixed inductance: the inductance of a coil on that core depends on μ, the cross‑section A and the magnetic path length l. Keep the three straight — H in A/m (field strength), B in T (flux density), Φ in Wb (total flux) — and remember the prefixes, since a 50 μT Earth field and a 50 mT motor gap differ by 1000.

13. Fields & material properties

Electric field strength E is measured in volts per metre (V/m), commonly prefixed to kV/m: it describes how strong an electric field is at a point, and appears in antenna and EMC work (a carrier at a few V/m is a strong near‑field), in PCB creepage/clearance calculations, and in electrostatics. It is distinct from voltage: voltage is a difference between two points (volts), while E is a field at a point (volts per metre). Likewise magnetic field strength H is in A/m, not amperes.

Material properties are quoted in per‑metre units and are easy to misread. Resistivity ρ is Ω·m and conductivity σ is S/m (σ = 1/ρ); a good conductor has tiny ρ and huge σ (copper ≈ 1.68×10⁻⁸ Ω·m, ≈ 5.8×10⁷ S/m). Dielectric and ferrite materials add relative permittivity εᵣ and relative permeability μᵣ (both dimensionless ratios), which scale capacitance and inductance. When a spec sheet gives a value in S/m or Ω·m, check the prefix — copper’s conductivity in MS/m (mega‑siemens per metre) is about 58, and dropping the “M” understates it by a million.

14. Time, temperature & physical dimensions

Time (unit the second, s) is prefixed ms, μs, ns and ps on schematics and in timing specs: gate delays are ns, setup/hold times are ns to ps, and an analog response might be quoted in μs or ms. Temperature uses three scales — kelvin (K, the SI base), degrees Celsius (°C) and Fahrenheit (°F) — with K = °C + 273.15 and °F = °C × 9/5 + 32, so absolute zero is 0 K = −273.15 °C = −459.67 °F. Component temperature coefficients are in ppm/°C, and derating curves are read in °C or K; note that Celsius and Kelvin share the same increment (a 10 °C rise is a 10 K rise) but not the same offset, so you add 273.15 rather than multiply.

Physical dimensions appear all over hardware. Length uses the metre with mm, μm and nm: PCB trace widths are mm to tens of μm, and IC feature sizes are nm. Area uses m² with mm², μm² (die area), and the imperial in² and the wire “mil²” (a mil is 0.001 in, so 1 mil² = 10⁻⁶ in²; the “circular mil” used in the AWG standard is π times a diameter squared in mils). Volume uses m³ with L and mL (1 L = 1000 mL = 1000 cm³). The recurring pitfalls are mixing a mil with a millimetre (1 in = 25.4 mm exactly, so 1 mil = 0.0254 mm), and treating a °C value as if it were K (a ratio vs an offset).

15. Confusing units & common pitfalls

Almost every unit error falls into one of three families: a prefix slip (k vs M, m vs M, μ misread as m), a dimension slip (treating a rate as an amount — W vs Wh, A vs Ah — or a magnitude as a component — VA vs W, VA vs var), and a scale slip (°C vs K offset, Hz vs rad/s, mil vs mm). The defense is a habit: before trusting any conversion, name the dimension of the quantity, check the prefix symbol’s case, and ask whether the unit is a rate, an amount, a magnitude, or a component. The comparison table below lists the pairs that cause the most real‑world mistakes.

A quick pre‑flight checklist: (1) Is the unit a base or a prefixed form, and which prefix? (2) Is this a rate (per second) or a total (over time)? (3) Is this the full magnitude or one component of a vector/complex quantity? (4) Does the quantity have an offset (temperature) rather than a pure ratio? (5) For AC, is power factor in play? Answering those five before you convert catches the vast majority of silent errors — and the interactive converter below gives you a second, arithmetic opinion on the result.

16. Best practice

Keep units attached to numbers at all times — write 4.7 kΩ, never a bare 4.7 — and convert to base units for intermediate math, then re‑express the final answer in a prefix that keeps the number between about 0.1 and 1000. Within a single design, pick one preferred prefix per quantity (kΩ for resistors, nF for decoupling, MHz for clocks) and stick to it, so a reader never has to mentally rescale from line to line. Dimensional analysis — checking that the base units (kg, m, s, A) balance on both sides of an equation — is a free correctness check that catches prefix and formula errors before simulation or fabrication.

Treat datasheets with a grain of salt on units: manufacturers sometimes omit the prefix, use engineering shorthand (“E3” for ×10³), or quote a typical value in a different prefix than the minimum/maximum. Convert the whole spec into one consistent unit before you design against it, and document any assumption. A silent unit error is the most expensive kind, because nothing fails loudly — the board simply does not meet spec, and the cause is a factor of ten hiding in a prefix. The tables, the converter and the self‑checking examples on this page are there to make that class of error rare.