How to Use This GMRS ERP & Coverage Calculator
- Select your station class. Repeaters, base stations, and mobiles have different FCC power limits under 47 CFR §95.1767.
- Enter transmitter output power — the rated wattage at the radio's antenna port or duplexer output, not the AC wall draw.
- Choose your coax type and run length. The tool uses verified loss figures at ~460 MHz for common cables. For custom cable, select "Custom" and enter the loss in dB per 30 m (100 ft).
- Enter connector and miscellaneous loss — each PL-259/SO-239 pair adds ~0.15–0.3 dB; surge arrestors add another 0.1–0.3 dB.
- Set antenna gain and unit (dBi or dBd). Most spec sheets use dBi; the tool converts automatically. A typical 5/8-wave vertical is about 5 dBi.
- Enter antenna heights for both the repeater/base and the mobile or handheld unit. Coverage is the sum of both radio horizons.
- Results update instantly. Check the FCC status badge and the power budget breakdown.
⚠️ This tool gives a flat-earth radio horizon estimate. Real-world coverage is reduced by terrain, buildings, trees, and atmospheric conditions. For mission-critical installations, use propagation modeling software (e.g., Radio Mobile, SPLAT!) with actual terrain data.
Formulas & Method
ERP (dBW) = P_tx_dBW + G_dBd − L_feed_dB
G_dBd = G_dBi − 2.15
P_tx_dBW = 10 × log₁₀(P_tx_watts)
ERP (W) = 10 ^ (ERP_dBW / 10)
EIRP (W) = ERP × 1.64 [or ERP_dBW + 2.15 → EIRP_dBW]
Radio horizon (each end):
d (km) = 3.57 × √h (m) [k = 4/3 Earth refraction factor]
Combined coverage radius:
R = d_repeater + d_mobile
ERP is the power radiated relative to a half-wave dipole (the legal reference for FCC Part 95 limits). EIRP is referenced to an isotropic radiator and is 2.15 dB higher. The radio horizon formula uses the standard 4/3 effective Earth radius to account for normal atmospheric bending of radio waves.
Frequently Asked Questions
What is ERP in GMRS radio? ▼
ERP (Effective Radiated Power) is the actual power your antenna system radiates, factoring in the transmitter output, all feedline and connector losses, and antenna gain relative to a half-wave dipole (dBd). For GMRS, ERP — not raw transmitter power — is the quantity that must comply with FCC Part 95 limits. A high-gain antenna can push you over the limit even at low transmitter power, while long lossy coax can significantly reduce your effective radiated power.
What is the FCC power limit for a GMRS repeater? ▼
Under 47 CFR §95.1767, GMRS repeater, base, and mobile stations may not exceed 50 watts transmitter output power. Fixed stations (communicating only with other fixed stations) are capped at 15 watts. The 462 MHz interstitial channels are limited to 5 W ERP, and the 467 MHz interstitial channels allow only 0.5 W ERP. These are transmitter output limits; FCC Part 95E does not separately set an ERP ceiling for the main 462 MHz repeater channels — the 50 W limit applies to transmitter output power directly.
How far can a GMRS repeater reach? ▼
GMRS operates at UHF (~462–467 MHz) and is essentially line-of-sight. Coverage depends almost entirely on antenna height. A repeater antenna at 30 m (100 ft) AGL reaches a radio horizon of about 19.6 km (12.2 mi). Add a 1.5 m handheld's horizon of ~4.4 km and you get roughly 24 km (15 mi) maximum flat-terrain range. Real-world terrain, buildings, and trees typically reduce this to 60–80% of the calculated horizon. Power and ERP affect signal quality within the coverage area but have minimal effect on extending the line-of-sight horizon.
Does coax cable loss really matter for GMRS? ▼
Yes, significantly. At 460 MHz, RG-58 loses about 6–8 dB per 30 m (100 ft), which cuts your transmitter power to roughly 25% before it even reaches the antenna. LMR-400 only loses ~1.5 dB per 30 m, delivering about 71% of transmitter power to the antenna. The difference between cheap and quality coax on a rooftop repeater can easily be 4–5 dB — equivalent to running 8× less power. For VHF/UHF installations, always use low-loss coax (LMR-400 or better) and keep the run as short as practical.
Do I need a license to operate a GMRS repeater? ▼
Yes. In the United States, any GMRS operation — including repeaters — requires a valid FCC GMRS license under 47 CFR Part 95E. Licenses are issued for 10-year terms, require no exam, cover the licensee and their immediate family members, and currently cost $35. A GMRS license does not permit linking repeaters over the internet or connecting to the public switched telephone network. See the FCC Universal Licensing System (ULS) to apply.
What is the difference between dBi and dBd antenna gain? ▼
Both describe antenna gain but use different reference antennas. dBi is measured relative to a theoretical isotropic radiator (equal radiation in all directions). dBd is measured relative to a half-wave dipole. Since a dipole has 2.15 dBi of gain over an isotropic radiator, dBd = dBi − 2.15. FCC ERP calculations use dBd. Most commercial antenna specs quote dBi, so you need to subtract 2.15 before using those numbers in ERP calculations — which this tool does automatically when you select dBi.
What is the radio horizon formula for GMRS? ▼
The standard practical formula is d (km) = 3.57 × √h(m), where h is antenna height above ground in meters and 3.57 incorporates the 4/3 effective Earth radius factor for normal atmospheric refraction. This gives the maximum line-of-sight distance for each station; the combined range is the sum of both horizons. In imperial units: d (miles) = 1.415 × √h(ft). Actual range depends heavily on terrain — hills, buildings, and dense vegetation all reduce it.
Regulatory reference: FCC Part 95 Subpart E (GMRS) — 47 CFR §95.1701–95.1787 (eCFR). This tool is an estimate for planning and education purposes only; consult the official FCC rules and a licensed RF engineer for compliance-critical deployments.