Link Budget Calculator
Link Budget Calculator
Transmit power, antenna gains, cable losses, path loss and receiver sensitivity added up into a received power and a link margin, with a verdict — plus the maximum range the budget reaches for the fade margin you ask for.
link budget
20 dBm into 1 dB of feeder and a 2 dBi antenna, 1 km at 2.4 GHz with 10 dB of extra margin, into a 9 dBi antenna, 1 dB of feeder and a -90 dBm receiver
The budget, as a single sum in decibels
Pr = EIRP − FSPL − Mextra + Gr − Lr
margin = Pr − Srx
FSPL = 20 log₁₀ d(km) + 20 log₁₀ f(MHz) + 32.4478
range at a target margin M: d = 10(EIRP + Gr − Lr − Mextra − Srx − M − 32.4478 − 20 log₁₀ f) / 20 km
- P t
- transmitter output power at its own connector, in dBm
- EIRP
- effective isotropic radiated power — what the transmitter would need to radiate in every direction equally to put the same power on the boresight. This is the number a regulator caps
- M extra
- everything free-space loss ignores. This is the input that carries the honesty of the whole calculation
- S rx
- receiver sensitivity: the power at which the receiver still meets its error rate. Worse at higher data rates
- margin
- how many decibels of fading the link can absorb before it stops working
Worked example
20 dBm into 1 dB of feeder and a 2 dBi antenna, 1 km at 2.4 GHz with 10 dB of extra margin, into a 9 dBi antenna, 1 dB of feeder and a -90 dBm receiver
EIRP = 20 − 1 + 2 = 21.00 dBm, which is 125.9 mW
Free-space path loss at 1 km and 2.4 GHz is 100.05 dB; with 10 dB of extra margin the path costs 110.05 dB
Pr = 21.00 − 110.05 + 9 − 1 = -81.05 dBm — 7.85 pW, or 19.8 µV across 50 Ω
Margin = -81.05 − (-90) = 8.95 dB
That is 1.05 dB short of the 10 dB fade margin asked for, so the budget reaches 885.9 m at that margin and 2.802 km with none at all
Every 6 dB found anywhere in this list doubles the range, because the path loss rises 6.02 dB per doubling
Where the decibels usually go, and what each one is worth
| Term | Typical size | What it does to the range | Notes |
|---|---|---|---|
| Transmit power | 0 to 30 dBm | +6 dB doubles it | Capped by regulation as EIRP, so the antenna gain counts against the same limit. |
| Transmit antenna gain | 0 to 24 dBi | +6 dB doubles it | Gain is narrowness. A 24 dBi dish has a beam a few degrees wide and has to be aimed. |
| Feeder loss, each end | 0.5 to 6 dB | −6 dB halves it | RG-58 loses roughly 1 dB per metre at 2.4 GHz. On the receive side it costs noise figure too. |
| Free-space path loss | 60 to 190 dB | the term being paid for | 6.02 dB per doubling of range, and the same again per doubling of frequency. |
| Extra path margin | 0 to 40 dB | −6 dB halves it | Foliage, walls, rain, body loss, fading. The one term nobody can look up for you. |
| Receive antenna gain | 0 to 24 dBi | +6 dB doubles it | Usually the cheapest decibels in the whole budget. |
| Receiver sensitivity | −70 to −148 dBm | −6 dB halves it | Roughly 3 dB better for each halving of the data rate; the SX1276 reaches −136 dBm at spreading factor 12 and 125 kHz. |
Adding it all up, and the one term that is a guess
A link budget is a single sum. Start at the transmitter’s output, subtract what the feeder eats, add what the antenna concentrates, subtract what the path takes, add the receiving antenna’s gain, subtract its feeder, and compare what is left against the receiver’s sensitivity. The difference is the link margin, and it is the only number that matters: it is how many decibels of fading the link can absorb before it stops working. Everything is in decibels precisely so that this is addition rather than multiplication.
EIRP, and why the regulator cares about it. Transmit power minus feeder loss plus antenna gain is the effective isotropic radiated power — what an isotropic antenna would have to radiate to put the same power along the boresight. Radio regulations almost always cap EIRP rather than transmitter power, which is why bolting a high-gain antenna onto a legal transmitter is not a legal way to go further. In most of Europe the 2.4 GHz ISM limit is 20 dBm EIRP and the 868 MHz limit 14 dBm ERP; check your own jurisdiction before designing to a number.
The extra margin input is the whole honesty of the page. The path loss here is free-space loss, which assumes nothing is in the way, nothing reflects and nothing absorbs. That is a fair description of a satellite downlink and of essentially no terrestrial link. So there is a separate input for everything free space leaves out, and it is the reader’s judgement, not a lookup: a clear outdoor path with good Fresnel clearance might want 10 dB; a path with trees in it 15 to 25 dB; a wall or two another 5 to 15 dB each; a handheld against a body 3 to 6 dB; rain on a 20 GHz hop tens of decibels for a few minutes a year. Put a real number there. A link budget with zero extra margin is not a conservative estimate, it is a fiction.
What the margin is for. Fading is not a fixed loss, it is a distribution. A link with 0 dB of margin works about half the time; one with 10 dB works most of the time; one with 20 dB is what you build when somebody will be cross if it stops. The usual design target for a fixed outdoor link is 10 to 20 dB, and the page will tell you how far the budget reaches while still holding whatever figure you ask for. It also prints the range with no margin at all, which is the distance at which the link works on a good day and fails on a bad one.
Cheap decibels and expensive ones. Because free-space loss rises 6.02 dB per doubling of range, every 6 dB found anywhere in the budget doubles the distance. Antenna gain at the receiving end is usually the cheapest 6 dB there is; shorter or better coax is next; halving the data rate buys about 3 dB of sensitivity and is often free. Raising transmit power is the most expensive option and the one most likely to be illegal. And remember that gain is narrowness — a 24 dBi dish has a beamwidth of a few degrees and has to be aimed and stay aimed.
The propagation term on its own, with its assumptions, the Fresnel clearance the path needs and the two-ray behaviour that takes over at ground level, is on the free space path loss calculator — this page uses exactly the same formula. For the powers themselves, the dBm to watts converter; for sizing the antennas, the antenna length calculator; and for the mismatch loss a badly tuned antenna adds to the budget, the VSWR and return loss converter. Use the figures from your part’s datasheet; typical values vary widely between manufacturers.
Frequently asked questions
What is a good link margin?
At least 10 dB for a fixed outdoor link, and 20 dB or more where an outage would matter. Under about 6 dB the link is a demonstration rather than a deployment — rain, a lorry, a person in the path or a little antenna drift will take it down. A margin below zero means the signal arrives under the receiver’s sensitivity and nothing gets through.
What is EIRP and why is it not the same as transmit power?
EIRP is transmit power minus feeder loss plus antenna gain: what an isotropic radiator would need to emit to match your signal along the boresight. Regulations cap EIRP, so a 20 dBm transmitter feeding a 9 dBi antenna through 1 dB of cable is radiating 28 dBm EIRP and is over the 20 dBm limit that applies on 2.4 GHz in much of the world.
How far will this link go?
The page prints two ranges: the distance at which the margin you asked for is still intact, and the distance at which the margin reaches zero. Both are free-space figures with your extra margin applied once, at the distance you entered — a real path’s excess loss grows with distance, so treat both as optimistic and re-run the budget at the range you actually care about.
Does the receiver’s sensitivity depend on the data rate?
Yes, strongly. Sensitivity improves by roughly 3 dB for each halving of the symbol rate, because the receiver’s noise bandwidth halves with it. That is why the SX1276 reaches −136 dBm at spreading factor 12 and 125 kHz while the same chip at spreading factor 6 and 500 kHz manages −111 dBm: 25 dB, worth a factor of nearly twenty in free-space range, bought entirely with time.
Why is my real link worse than this calculation?
Almost always because the extra margin input was too small. Free-space loss is a floor; real paths at ground level lose considerably more, and past the two-ray breakpoint the loss climbs at about 40 dB per decade instead of 20. Antenna misalignment, polarisation mismatch, feeder faults and a noisy site all cost decibels the budget never saw.
Should the antenna gain be in dBi or dBd?
dBi, which is what this page expects. dBd is referenced to a half-wave dipole, which is itself 2.15 dBi, so add 2.15 to convert dBd to dBi. Antenna marketing quotes whichever is larger, so check which one a figure is.
Related calculators
References
- Recommendation ITU-R P.525-4 (08/2019), Calculation of free-space attenuation — the propagation term this budget uses: Lbf = 32.4 + 20 log f(MHz) + 20 log d(km) dB.
- Friis HT. A Note on a Simple Transmission Formula. Proceedings of the IRE, vol. 34 no. 5, May 1946, pp. 254–256. The received-power relation Pr = Pt Gt Gr (λ/4πd)² that this page’s decibel sum is the logarithm of; the arithmetic here was checked against that equation computed in watts.
- ETSI EN 300 328 V2.2.2 (2019-07), Wideband transmission systems; data transmission equipment operating in the 2,4 GHz band — the 20 dBm EIRP limit; and ETSI EN 300 220-2 for the 25 mW (14 dBm) ERP limit in the 863–870 MHz band.
- Semtech. SX1276/77/78/79 datasheet, rev. 7 (May 2020), §2.5.5 table 10, LoRa receiver specification: sensitivity −111 dBm at 500 kHz bandwidth and spreading factor 6, −136 dBm at 125 kHz and spreading factor 12, and −148 dBm at 7.8 kHz and spreading factor 12. The datasheet states about 3 dB per spreading-factor step at constant bandwidth, which is where the “3 dB per halving of the rate” rule of thumb on this page comes from.
- Rappaport TS. Wireless Communications: Principles and Practice, 2nd ed. Prentice Hall, 2002. Chapter 4 on propagation and chapter 3 on link budgets and fade margin — why margin is a statistical quantity and not a fixed loss.