RF Link Budget Calculator with Free-Space Path Loss
Enter the frequency, the distance, the transmit power and the antennas, and get the free-space path loss, the received power and the link margin against your receiver's sensitivity. It also shows EIRP, how far the link reaches in free space, the first Fresnel zone to keep clear, and a dBm to watts converter. Free space is the best case, so the page says plainly when a margin is too thin for a real link. Every input lives in the URL, so a copied link reproduces the calculation.
Each preset loads example antenna gains, losses and a sensitivity typical of that kind of link. Replace them with the figures from your own datasheets.
- Free-space path loss
- 105.6 dB
- 915 MHz over 5 km
- Received power
- -87.6 dBm
- 1.72 pW at the receiver input
- EIRP
- 16.0 dBm
- 39.8 mW: Tx power + antenna gain - cable loss
- Receiver sensitivity
- -137.0 dBm
- noise floor -117.0 dBm - 20 dB SNR
- Margin over fade allowance
- +29.4 dB
- 20 dB held in reserve
- Max free-space range
- 1470 km
- where the margin reaches zero
- Range at fade allowance
- 147 km
- free space, keeping 20 dB in hand
- First Fresnel zone
- 20.2 m
- radius at mid-path; keep 60% (12.1 m) clear
In free space this link closes out to 1470 km, 295× this path, and to 147 km with the 20 dB fade allowance held back. Every 6 dB of margin doubles free-space range. Near the ground, loss grows faster than free space, so trust the second figure more than the first, and a field test more than both. On the ground, the horizon and terrain end a link long before free-space loss does at these distances; the figure is literal only for air and space links.
The link reproduces these exact inputs - paste it in a design review or a pull request.
Bring the radio, the antennas and the range you need. We will go through the budget with you (antenna choice, matching, feed-line loss, the regulatory ceiling) before the board goes to fab. If you need the radio board done, see SDR and RF board design services.
dBm to watts converter
dBm is power in decibels relative to one milliwatt: P(mW) = 10^(dBm / 10), and dBm = 10 log10(P in mW). Type in either box and the other follows.
30.00 dBm = 1 W = 0.00 dBW
| dBm | Power | Where you meet it |
|---|---|---|
| 0 dBm | 1 mW | The reference level: 1 mW. A common Bluetooth Low Energy default. |
| 10 dBm | 10 mW | A strong Bluetooth Low Energy or Zigbee transmitter. |
| 14 dBm | 25.1 mW | A common LoRa setting; 25 mW is the limit in most of the EU 868 MHz band. |
| 20 dBm | 100 mW | A typical Wi-Fi client, and the EU 2.4 GHz EIRP limit. |
| 27 dBm | 501 mW | About half a watt: a high-power sub-GHz module or a small PA. |
| 30 dBm | 1 W | The FCC Part 15.247 conducted limit in 902 to 928 MHz and 2.4 GHz. |
| 36 dBm | 3.98 W | About 4 W: the Part 15.247 EIRP cap for point-to-multipoint links. |
Free-space path loss by band and distance
Best-case loss between two antennas with clear line of sight, from the same equation the calculator uses. Ten times the distance adds 20 dB; so does ten times the frequency.
| Frequency | 10 m | 100 m | 1 km | 10 km |
|---|---|---|---|---|
| 433 MHz | 45.2 dB | 65.2 dB | 85.2 dB | 105.2 dB |
| 868 MHz | 51.2 dB | 71.2 dB | 91.2 dB | 111.2 dB |
| 915 MHz | 51.7 dB | 71.7 dB | 91.7 dB | 111.7 dB |
| 2.4 GHz | 60.0 dB | 80.0 dB | 100.0 dB | 120.0 dB |
| 5.8 GHz | 67.7 dB | 87.7 dB | 107.7 dB | 127.7 dB |
The equation
A link budget is bookkeeping in decibels: start with the transmit power, add every gain, subtract every loss, and compare what arrives with what the receiver needs. Free-space path loss is FSPL(dB) = 20 log10(d in km) + 20 log10(f in MHz) + 32.44. Received power is Prx(dBm) = Ptx + Gtx + Grx - FSPL - other losses, and link margin is Prx minus the receiver sensitivity. Path loss is not absorption. The power spreads over a growing sphere, and an antenna of fixed gain collects from a smaller area as the wavelength shrinks, which is why doubling either the distance or the frequency costs 6 dB.
Free space is the best case
The equation describes two antennas with nothing near the path. Every real link is worse: the ground reflects a second copy of the signal that can cancel the first, buildings and trees block part of the wavefront, and indoor walls and floors each take their own share. Close to the ground, past a breakpoint set by the antenna heights, loss grows nearer 40 dB per tenfold distance than the 20 dB of free space. That gap is what the fade allowance is for, and why the calculator reports range both at zero margin and with the allowance held back. Line of sight also needs room around it: the first Fresnel zone has a radius of r(m) = 8.657 × sqrt(d in km / f in GHz) at mid-path, and about 60% of it should be clear. On a 5 km path at 915 MHz that radius is 20.2 m, so 12.1 m of clearance in the middle. Antenna height often buys more than transmit power does.
Where sensitivity comes from
A receiver's floor is thermal noise: -174 dBm in every hertz at room temperature, plus 10 log10 of the bandwidth, plus the receiver's noise figure. Sensitivity is that floor plus the signal-to-noise ratio the modulation needs. A 1 MHz channel with a 6 dB noise figure has a floor of -108.0 dBm; if the demodulator needs 10 dB of SNR, the sensitivity is -98.0 dBm. Narrow the channel to 125 kHz and the floor drops to -117.0 dBm, and a spread-spectrum mode that decodes 20 dB below the noise reaches -137.0 dBm. That is the trade behind every long-range radio: ten times less bandwidth buys 10 dB, which is 3.16× the free-space range, and costs ten times the airtime.
EIRP and the regulatory ceiling
EIRP is what the transmitter radiates in its strongest direction: transmit power plus antenna gain minus the cable loss between them. Regulators cap it, so a bigger transmit antenna is not free range. As general guidance, not legal advice: under FCC Part 15.247 the 902 to 928 MHz and 2.4 GHz bands allow up to 30 dBm conducted into a 6 dBi antenna, which is 36 dBm EIRP, for point-to-multipoint systems, and the conducted power has to come down when the antenna gain goes above 6 dBi. Fixed point-to-point links at 2.4 GHz and 5.8 GHz are treated more generously, and other regions are much stricter. Check the rule part for your band, modulation and antenna. Gain at the receive end is never counted, so a better receive antenna or a lower noise figure is the margin no rule takes away.
Frequently asked questions
How do I calculate a link budget?
Work in decibels and it is a sum: received power (dBm) = transmit power + transmit antenna gain + receive antenna gain - free-space path loss - every other loss (cables, connectors, polarisation, enclosure). Link margin is the received power minus the receiver's sensitivity. Example: a 0 dBm radio with 0 dBi antennas at both ends, at 2.4 GHz over 100 m. The path loss is 80.0 dB, so -80.0 dBm arrives. Against a -95 dBm receiver that is 15.0 dB of margin: enough for clear line of sight, thin for indoors, where 20 dB or more is the usual target.
What is the free-space path loss at 2.4 GHz over 100 m?
80.0 dB. FSPL = 20 log10(d in km) + 20 log10(f in MHz) + 32.44, and for 0.1 km at 2400 MHz that is -20 + 67.6 + 32.44. Every tenfold change in distance moves it 20 dB (60.0 dB at 10 m, 100.0 dB at 1 km) and every doubling adds 6 dB (86.1 dB at 200 m). Over the same 100 m, 915 MHz loses 71.7 dB and 5.8 GHz loses 87.7 dB. These are best-case figures for clear line of sight; walls, floors and people add to them.
How many watts is 30 dBm?
1 W. Power in milliwatts is 10^(dBm / 10), so 30 dBm is 1000 mW. Every 10 dB is a factor of ten and every 3 dB is very nearly a factor of two: 0 dBm is 1 mW, 20 dBm is 100 mW, 27 dBm is 501 mW, 33 dBm is 2 W and 36 dBm is 3.98 W. Going the other way, dBm = 10 log10(P in mW), so 2 W is 33.0 dBm. The converter on this page works in both directions.
How far can LoRa reach at 14 dBm?
In free space, much farther than any real link. With 14 dBm, 2 dBi antennas at both ends and a receiver at -137.0 dBm (125 kHz bandwidth, 6 dB noise figure, spreading factor 12 decoding 20 dB below the noise), the link survives 155.0 dB of path loss. At 915 MHz that is 1470 km of free space, or 147 km with a 20 dB fade allowance held back. No link on the ground gets near either figure: terrain, buildings, the horizon and a first Fresnel zone 20.2 m in radius at the middle of a 5 km path decide the range. Expect a few kilometres in a city and ten or more with clear line of sight from a high gateway. At spreading factor 7 the sensitivity is -124.5 dBm and the free-space figures fall to 349 km and 34.9 km.
What link margin do I need?
Enough to cover everything the free-space equation leaves out. Below 0 dB there is no link. Below 10 dB a link is fragile: it works on the bench and drops when someone stands in the path. Mobile and indoor links usually target 20 dB or more, because multipath fades of that depth are routine. A fixed link with clear line of sight and a clear Fresnel zone can run on 10 to 15 dB, and a space link is budgeted tighter still because its path really is free space. Margin converts directly to range: every 6 dB doubles the free-space distance, and 20 dB is a factor of ten.
If the radio is the product
A link budget says whether a link can close. Whether it does is decided on the board: the match between the radio and the antenna, the feed line, the ground plane the antenna needs and the enclosure around it can each take several dB that no spreadsheet shows. That is our day job: SDR and RF board design explains how we work, and CubeSat payload electronics covers the links that really are free space. For the 50 Ω feed line itself, the microstrip impedance calculator is next door, and because every dB of transmit power is paid for by the battery, so is the battery life calculator. If a number here decides something real, a free 30-minute technical call is the fastest way to check it against someone who has shipped the radio.