RF Link Budget Calculator: EIRP, FSPL, Rx Power & Margin

RF Link Budget Calculator

Work out whether a wireless link closes. Enter transmit power, antenna gains, cable losses, distance, and frequency, and this tool computes EIRP, free space path loss (FSPL), the received signal power at the far end, and the link margin against your receiver sensitivity so you know if the path has enough spare decibels.

📶Real Link Presets

📡Link Inputs

Transmit power Ptx (dBm)

Power at the radio output. 20 dBm = 100 mW, 30 dBm = 1 W.

Tx antenna gain Gtx (dBi)

Directional gain of the transmit antenna in dBi.

Tx cable and connector loss (dB)

Feedline plus connector loss on the transmit side.

Distance

Line of sight path length between the two antennas.

Distance unit

Applies to the distance value above.

Frequency

Center frequency of the RF carrier.

Frequency unit

Applies to the frequency value above.

Rx antenna gain Grx (dBi)

Directional gain of the receive antenna in dBi.

Rx cable and connector loss (dB)

Feedline plus connector loss on the receive side.

Extra and atmospheric loss (dB)

Rain, foliage, polarization, and pointing losses combined.

Receiver sensitivity (dBm)

Minimum usable signal for the chosen data rate, negative.

Required fade margin (dB)

Spare margin target for reliability, often 10 to 20 dB.

EIRP 0 dBm Ptx + Gtx − Tx loss

Free Space Path Loss 0 dB spreading loss over distance

Received Power 0 dBm signal at receiver input

Link Margin 0 dB Prx − sensitivity

Link closes

🔢Formula Snapshot

EIRPPtx + Gtx − L

FSPL20log d + 20log f + 32.44

PrxEIRP + Grx − L − FSPL

MarginPrx − sensitivity

📋Path Loss by Distance and Frequency

Distance	Frequency	FSPL (dB)	Reads As
100 m	2400 MHz	80.1 dB	Indoor hop
1 km	900 MHz	91.5 dB	Short UHF
1 km	2400 MHz	100.1 dB	Wi-Fi PtP
1 km	5800 MHz	107.7 dB	5 GHz bridge
5 km	915 MHz	105.7 dB	LoRa town
10 km	5800 MHz	127.7 dB	Long 5 GHz
40 km	150 MHz	108.0 dB	VHF range
100 km	14 GHz	155.3 dB	Microwave

🔌Transmit Power and Antenna Reference

Item	Value in dB	Linear Equivalent	Where Seen
Tx power	0 dBm	1 mW	Weak beacon
Tx power	20 dBm	100 mW	Wi-Fi radio
Tx power	30 dBm	1 W (1000 mW)	PtP max EIRP feed
Antenna	0 dBi	Isotropic ref	Ideal point source
Antenna	2.15 dBi	Half-wave dipole	Rubber duck
Antenna	24 dBi	~250x gain	Parabolic dish
Cable	3 dB loss	Halves the power	Long coax run

📶Typical Receiver Sensitivity

System	Band	Data Rate	Sensitivity	Note
Wi-Fi 802.11n	2.4 GHz	MCS0 6.5 Mbps	-90 dBm	Robust rate
Wi-Fi 802.11ac	5.8 GHz	MCS9 866 Mbps	-59 dBm	Top rate
LoRa SF12	915 MHz	~290 bps	-137 dBm	Long range
LTE Cat-1	1800 MHz	10 Mbps	-97 dBm	Cell edge
GPS L1 C/A	1575 MHz	50 bps	-130 dBm	Below noise
Zigbee	2.4 GHz	250 kbps	-95 dBm	IoT mesh
FM voice	150 MHz	Analog NBFM	-116 dBm	12 dB SINAD

🗃Full Link Budget Comparison Grid

Link	EIRP	Distance	Freq	FSPL	Rx Power	Margin
Wi-Fi 2.4 PtP	30.5 dBm	5 km	2400 MHz	114.1 dB	-76.1 dBm	+13.9 dB
Wi-Fi 5.8 bridge	34 dBm	10 km	5800 MHz	127.7 dB	-71.7 dBm	+18.3 dB
LoRa 915	16 dBm	15 km	915 MHz	115.2 dB	-105.2 dBm	+31.8 dB
LoRa 868	14 dBm	10 km	868 MHz	111.2 dB	-105.2 dBm	+31.8 dB
Cellular 1800	58 dBm	3 km	1800 MHz	107.1 dB	-51.1 dBm	+45.9 dB
14 GHz microwave	66 dBm	30 km	14 GHz	144.9 dB	-84.9 dBm	+5.1 dB
VHF 150 voice	44 dBm	40 km	150 MHz	108.0 dB	-67.0 dBm	+49.0 dB
UAV 2.4 control	26 dBm	2 km	2400 MHz	106.1 dB	-83.1 dBm	+11.9 dB
Sat IoT S-band	20 dBm	780 km	2200 MHz	157.1 dB	-134.1 dBm	+2.9 dB
GPS L1	27 dBm	20200 km	1575 MHz	182.5 dB	-152.5 dBm	-22.5 dB

⚙Formula Breakdown

EIRP = Ptx + Gtx − LtxEffective isotropic radiated power. A 20 dBm radio with a 12 dBi antenna and 1.5 dB cable loss gives EIRP = 20 + 12 − 1.5 = 30.5 dBm.

FSPL = 20log10(d) + 20log10(f) + KFree space path loss. With d in km and f in MHz, K = 32.44. For d in km and f in GHz use K = 92.45. Loss grows 6 dB each time distance or frequency doubles.

Prx = EIRP + Grx − Lrx − FSPL − LmiscReceived power adds receive antenna gain, then subtracts receive cable loss, path loss, and atmospheric or misc loss from the EIRP.

Margin = Prx − SensitivityLink margin is how far the received signal sits above the receiver sensitivity. A margin of 0 dB is the bare edge of decoding.

Closes when Margin ≥ FadeA robust link keeps a fade margin of 10 to 20 dB above zero to survive rain, multipath, and pointing drift. Below the fade target the link is marginal.

6 dB rule of thumbDoubling range or frequency adds 6 dB of path loss; doubling antenna area or power adds about 3 dB of link budget. Use this to trade parts quickly.

💡Link Planning Tips

Budget a real fade margin: A link with just +2 dB of margin looks like it closes on paper but drops out in rain or wind. Design for 10 to 20 dB of spare margin. On a 5.8 GHz 10 km bridge at 34 dBm EIRP the received power near -72 dBm against a -90 dBm sensitivity leaves +18 dB, which is comfortable through weather.

Watch cable loss at high MHz: Coax loss climbs sharply with frequency. A 30 m run of RG-58 loses roughly 3 dB at 100 MHz but over 20 dB at 2400 MHz, wiping out an antenna gain. Keep feedlines short, use low-loss cable like LMR-400, or mount the radio at the antenna to protect the link margin.

When two radios talk to each other over a radio link, they use a running tally of decibels called a radio frequency link budget. This budget accounts for everything that adds to or subtracts from a signal during it’s trip from one radio to another. This RF link budget calculator do that math for you. Plug in the frequency, distance, transmit power, antenna gain, cable losses, etc., and it spits back four numbers that determine if your wireless link will close successfuly.

If the number is comfortabley positive, then there’s enough power to get the job done. If it’s negative, no amount of wishful thinking will help. That budget, that running tally in decibels… Is what every wireless system lives or dies by. Transmitters can only push so much power (under regulatory caps) and receivers can only pull a signal out of noise down to some fixed sensitivity limit. In between those limits lies the path, which steal decibels via weather, distance, frequency, and cables.

How to Calculate Your Wireless Signal Strength

How do we calculate that? The first result is EIRP, effective isotropic radiated power. In other words, it tell you how much power your antenna system actualy sends out in its best direction compared to a hypothetical transmitter that sends the same amount in all directions. It’s the radio output + the transmit antenna gain, the loss from any cabling linking those two together. If you have a 20 dBm radio powering a 12 dBi antenna via a 1.5 dB of cabling, then you get an EIRP of 30.5 dBm. This is what you’re effectively launching off-site. Regulators are frequently interested in this value so they can ensure you don’t interfere with others’ networks.

Free space path loss, the next item in the list, is typically the largest contribution to the entire formula. Since a radio wave is spread out as it travels, its power density decreases according to the inverse of the square of distance traveled, that’s what it means for power to fall off at a rate of R-squared. In other words, every doubling of distance (or frequency) add another 6 dBs of loss. This is also why antennas must be larger for high-frequency links; they has to get more signal into the air to achieve comparable ranges. Plug in your own carrier frequency and hop length, and the calculator does the math for you. You won’t have unit mismatches or should of had to do logarithmic conversions by hand.

Third is what we call “received power”. I.e., how much signal is actualy at your receiver’s input. This value begins with your EIRP and includes gain of your receive antenna. It then subtracts out path loss, receive cable loss, and any additional atmospheric loss due to foliage or rain. Because the signal is so small by the time it gets to you, the resulting number in dBm is nearly always negative. For example, a received power of -76 dBm is a healthy level for most Wi-Fi radios. Depending on your modulation scheme and data rate, other systems may find that barely noticeable.

That brings us to the last card: the link margin. This is the key for long term reliability. Simply put it’s the received power minus the receiver sensitivity. With 14 dB of margin, the connection will work. At zero, it’s just the ragged edge, a tiny bit of disturbance and down we go. Engineers say the fade margin should be at least 10 to 20 dB to withstand both antenna drift and rain fades. Does your set up clear that? That’s what the tool flags. It tells you if the link is marginal or strong.

Real-world installations use mixed units. Because of this, the tool accepts frequency input in either gigahertz or megahertz, and distance can be input in kilometers or miles. The software converts internally and applies formula for loss. This prevents errors from mixing metric frequencies used on radios with imperial measures used in site surveys. It includes built-in presets covering backhauls by microwaves, downlinks from orbit by GPS, and real systems such as LoRa nodes trading sensitivity for data rates. When you load one of these presets, you immediately get a sense of how various links compare.

For example, the GPS case is particularly helpful: Its margin appears negative when compared to simple sensitivity numbers. True navigation requires processing gain which isn’t reflected in a mere budget number. Feed it honest numbers if you want to believe what comes out. Get antenna gains from the datasheet, not a marketing brochure. The actual coax length and type at your operating frequency matter; coax loss increases steeply with frequency, and can eat away at your antenna gains slightly on long runs. Receiver sensitivity reflects the data rate you really require, not the fastest that the radio can support.

If applied carefully, it translates a bunch of disconnected specs to one clear answer: Will your link work or not? It eliminates guessing at antenna sizes and distance planning. It converts airy-fairy dreams to concrete protection from fade and interference. It makes sure your signal reaches its destination, doesn’t merely get started there.