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Radio Tools

Radio Calculators

Explore the Numbers Behind Radio

Working out a wavelength, planning an antenna, or learning how a circuit works? These calculators help you explore the relationships behind the numbers.

Enter your values, see the result, and follow the calculation step by step. You don’t have to memorize every formula to start learning.

1 Start here

Wavelength & Antenna Length

Calculate full, fractional, and multiple wavelengths from a frequency. Compare common lengths used in antenna designs and learn how the calculation works.

Result

Enter a frequency and press Calculate. Results appear here.

How It Works

Wavelength is the distance a wave travels during one complete cycle. Divide the speed of light by the frequency to find the full wavelength, then multiply by the fraction you want.

Full wavelength in meters = 299,792,458 ÷ frequency in Hz

Selected length = full wavelength × selected wave fraction

For a quick estimate using MHz:

Quick approximation only Wavelength in meters ≈ 300 ÷ frequency in MHz.

A quarter wave is one-fourth of that distance, a half wave is one-half, and a five-eighths wave is five-eighths.

c
the speed of light (radio waves) in a vacuum: exactly 299,792,458 meters per second. The calculator always uses this exact value.
f
frequency: how many cycles happen each second, in hertz (Hz).
λ
“lambda”, the wavelength: the length of one complete cycle.

Radio waves travel through a vacuum at 299,792,458 meters per second. Dividing that speed by the frequency tells us the distance covered during one cycle.

These are free-space wavelengths. Signals travel more slowly through materials such as coaxial cable.

Example

At 146 MHz, the calculated free-space lengths are approximately:

  • Full wave: 2.053 meters
  • ¼ wave: 0.513 meters
  • ½ wave: 1.027 meters
  • ⅝ wave: 1.283 meters

That helps explain why this frequency is described as part of the 2-meter band.

Select a different frequency to see how these lengths change.

Understanding Antenna Length

These calculations give the free-space length for each wave fraction—a useful reference when exploring antenna designs.

An antenna’s finished physical dimensions also depend on its design, materials, element diameter, loading, and surroundings. Use the calculated length as a reference, then follow the construction and tuning guidance for your chosen antenna.

2 Electrical

Ohm’s Law

Ohm’s law describes the relationship between voltage, current, and resistance. If you know two, you can calculate the third.

Result

Choose what to find, enter the other two values and press Calculate.

The formulas

Voltage: V = I × R

Current: I = V ÷ R

Resistance: R = V ÷ I

V
voltage, in volts (V).
I
current, in amperes (A). 1 milliampere (mA) is one-thousandth of an ampere.
R
resistance, in ohms (Ω). 1 kilohm (kΩ) is 1,000 ohms.

Voltage is the electrical potential difference between two points. Current is the flow of electric charge. Resistance describes how strongly a component opposes that flow.

For a fixed resistance, increasing the voltage increases the current. For a fixed voltage, increasing the resistance decreases the current.

This calculator uses a simple resistive model. AC and radio-frequency circuits can also involve reactance and impedance.

Example

With 12 volts across a 100-ohm resistor:

I = V ÷ R
I = 12 ÷ 100
I = 0.12 amperes, or 120 milliamperes.

3 Electrical

Electrical Power

Electrical power describes the rate at which electrical energy is transferred. In a DC circuit, voltage multiplied by current gives power in watts.

Result

Enter a voltage and a current and press Calculate.

The formula

P = V × I

P
power, in watts (W).
V
voltage, in volts (V).
I
current, in amperes (A).

Example

A device drawing 2 amperes from a 12-volt supply uses:

P = 12 × 2
P = 24 watts.

Note

This result is electrical input power. It does not tell you a transmitter’s RF output power or how much power an antenna radiates. AC circuits may require additional information.

4 Antennas

Dipole Starting Length

Estimate a starting length for a conventional half-wave wire dipole. A dipole has two legs, with the feed point between them.

Result

Enter a frequency in MHz and press Calculate.

The rule of thumb

Total length in feet = 468 ÷ frequency in MHz

Each leg = total length ÷ 2

This practical rule of thumb estimates a wire dipole’s physical length. It differs from simply dividing the free-space wavelength in half.

Wire construction, antenna height, nearby objects, and installation details affect the final resonant length. Leave extra wire for adjustment, then measure and tune the installed antenna.

This estimate is for a conventional half-wave wire dipole only. It isn’t a formula for other antenna types or for coaxial cable length.

Example

At 14.2 MHz:

Total length ≈ 468 ÷ 14.2 ≈ 32.96 feet.
Each leg ≈ 16.48 feet.

These are starting estimates, not guaranteed finished dimensions.

Formula Sources

Independent references for the formulas and constants used on this page. The explanations above were written for this site.

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