Predict WiFi Signal Through Brick Walls — A No-Survey Guide

The basic link budget

Received signal power, in dBm, is:

Pr = EIRP - PL_free_space - sum(wall_attenuations)

Three pieces:

• •EIRP — effective isotropic radiated power, in dBm. Sum of AP transmit power and antenna gain. A typical consumer AP at 20 dBm Tx with a 3 dBi omni antenna gives 23 dBm EIRP.
• •PL_free_space — free-space path loss in dB, computed from frequency and distance (Friis equation, log form).
• •sum(wall_attenuations) — the dB cost of every wall the signal crosses, summed. This is the "multi-wall" model.

That's the whole COST-231 multi-wall model in one line. Real RF planners add a tweak factor for the number of walls (the "non-linear wall correction") but for ≤4 walls the additive approximation is within a couple of dB.

Free-space path loss, properly

The free-space path-loss formula in log form, using metres and MHz:

PL(dB) = 20 * log10(d_m) + 20 * log10(f_MHz) - 27.55

This is just Friis' equation, rearranged so distance is in metres and frequency in MHz. Some textbooks write it with km and GHz; the constant changes accordingly. For WiFi work, the metres-and-MHz form is most convenient.

Quick reference values you can memorise:

Two practical observations: doubling the distance costs you 6 dB; doubling the frequency also costs 6 dB. That's why 5 GHz is "shorter range" than 2.4 GHz — same 6 dB penalty for operating up the band.

Wall attenuation — ITU-R P.2040

ITU-R P.2040 is the recommendation that catalogues the electrical properties of building materials at common radio frequencies. From it, the typical 5 GHz attenuation values used by every COST-231-style planner (including ours, Ekahau's entry tier, and Hamina) are:

These are typical values. Actual attenuation depends on wall thickness, moisture content, and what's embedded (pipes, electrical conduit). Treat them as ±2 dB.

Worked example: 5 GHz AP, 8 m, one brick wall

Let's run the actual math. We have a UniFi-style AP at 20 dBm Tx, 3 dBi antenna, so:

EIRP = 20 dBm + 3 dBi = 23 dBm

The receiver (a phone) is 8 m away. Frequency is 5 GHz = 5000 MHz. Plug into the formula:

PL_free_space = 20 * log10(8) + 20 * log10(5000) - 27.55
              = 20 * 0.903 + 20 * 3.699 - 27.55
              = 18.06 + 73.98 - 27.55
              = 64.49 dB

One brick wall in the path adds 8 dB:

Total path loss = 64.49 + 8 = 72.49 dB

Pr = EIRP - PL = 23 - 72.49 = -49.5 dBm

−49 dBm on the far side of one brick wall at 8 m. That's a solid signal — green band, full speed link. Add a second brick wall (15 m diagonal across a small apartment) and you're at roughly −58 dBm, still fine. Two brick walls and 15 m starts to bite: closer to −65 dBm, which is workable but not great.

This is exactly the math our tool runs at every pixel of the heatmap — for each AP, for each pixel, compute distance to the AP, sum the wall attenuations on the line segment between them, and take the strongest of the per-AP results.

What this model misses

I want to be upfront about the limits of the additive multi-wall model. There are at least four things it doesn't represent, and you should know what they are before you trust the heatmap to within 2 dB:

• •Diffraction. Signals bend around wall edges. The model treats every wall as an infinite slab in the path; real walls end at doorways. Through a doorway your real attenuation is closer to free-space than to "brick".
• •Multipath. Signals also bounce off floors, ceilings, mirrors, and metal appliances. Sometimes they constructively interfere (you get a stronger signal than predicted); more often they fade (Rayleigh / Rician). Real-world variance from multipath is ±3–6 dB even in a static environment.
• •3D effects. A 2D planner like ours treats the floorplan as if signals only travel horizontally. Ceiling-mounted APs actually radiate spherically; ground-floor APs lose signal into the floor; upstairs APs lose signal into the floor below. For a single-floor apartment this is fine. For a 3-storey house, it's noticeably wrong.
• •Antenna patterns. COST-231 assumes isotropic radiation. Real antennas have lobes — UniFi U7 Pro is roughly omni-horizontal but has a null directly above and below the AP. Directional antennas (sector, panel, dish) need a completely different model.

Net of all that, expect the predictive heatmap to be within ±6–10 dB of real-world measurements in a typical home or small office. That's enough to make placement decisions confidently. It's not enough to certify a survey for a regulated environment.

When to stop computing and just measure

Predictive planning is for the placement decision: do I need 1 AP or 2? Where do I put them? Is the bedroom going to be a dead zone? Once the APs are installed, you validate empirically with a phone-based survey app — the prediction tells you where to check, and reality tells you what's actually there. Both halves matter.

If you'd rather not do the arithmetic by hand, the editor runs exactly the formulas above at every pixel of your floorplan, with the wall attenuation values from the table above, and updates live as you drag APs around. It's free for up to 5 APs and runs entirely in your browser.