dBsm means decibels relative to one square metre: σdBsm = 10·log10(σ / 1 m²). 0 dBsm is 1 m², and every 10 dB more is ten times the area. A person is about 0 dBsm, a bird −20 to −30, a jumbo jet +20 and a 5,000 t ship +40. The log scale is practical because typical targets span about 90 dB, from an insect at −50 dBsm to a ship.

Why can a small corner reflector look as big as a boat?

Because it sends the energy straight back to the radar from a wide range of angles. Three plates at right angles bounce a ray three times and return it parallel to its path. Its RCS grows with the fourth power of its size and with frequency squared, 4π·a⁴/(3λ²). A 30 cm triangular reflector reaches 37.75 m² at 10 GHz, more than a 40–50 ft cabin cruiser at 10 m². With a 1.5 m edge it reaches about 23 600 m², more than a 5,000 t ship.

Why does RCS depend on the wavelength?

Because a flat or corner-shaped reflector focuses its echo into a beam about λ/a wide. A shorter wavelength makes that beam narrower and stronger. That is the λ² in the plate and trihedral formulas, so doubling the frequency gives 4 times the RCS (+6 dB). A sphere scatters evenly and stays at π·a² in the optical region. Objects much smaller than the wavelength fall into the Rayleigh region, where the RCS drops as 1/λ⁴. That is why insects nearly vanish for long-wavelength radars.

Why do the shape formulas only work for large objects?

Because they assume the optical region, where the object is much larger than the wavelength (ka ≳ 10). Near ka ≈ 1 the echo interferes with waves creeping round the object and the RCS oscillates. Below that it falls steeply. The calculator flags both cases. It also leaves out the dihedral and the circular trihedral, whose formulas could not be checked in a primary source.

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Radar cross section (RCS) calculator: m² ↔ dBsm

Q: Why does a car show 100–200 m² here when automotive radar uses about 10 m²?

Because both figures are in use and neither is wrong for its context. Skolnik’s table gives 100–200 m² for an automobile or small truck at microwave frequencies. Automotive radar at 77 GHz commonly works with about 10 m² (10 dBsm). We found no authoritative open source for that practice figure, so the table labels it as such. Either way the RCS of a real car changes by 10 dB or more with the viewing angle.

Radar cross section (RCS) is how large a target looks to a radar, as the area of a perfect reflector that would return the same echo. It is given in m² or in dBsm, decibels relative to 1 m². This calculator converts between the two, compares the value with typical targets and gives the RCS of spheres, flat plates and corner reflectors.

Convert m² ↔ dBsm

Radar cross section

Type a value in m², dBsm, ft² or cm². The unit picker converts the value in place.

Typical target

A selection of ten typical values, read 7 November 2026. Choosing one fills in the middle of its range in dB.

Converted

0.00

pick another unit on the card

Sphere with the same RCS

1.128

diameter, optical region (σ = π·d²/4)

Comparable to

person

typical person: 1 m² · also fighter

Detection range vs 1 m²

×1

same radar, range ∝ ∜σ

Simple shapes (optical region)

Peak RCS of perfectly conducting shapes, valid only when they are much larger than the wavelength (ka ≫ 1).

Shape

Size a

Radius of the sphere, side of the plate, or edge length of the trihedral along its open edges.

Radar frequency

Sets the wavelength. The sphere does not depend on it, the other shapes grow with frequency squared.

RCS of the shape

37.75

4π·a⁴/(3λ²), peak along the axis

Electrical size ka

62.9

optical region, the formula applies

Wavelength

2.998

λ = c / f

Comparable to

airliner

typical airliner: 40–100 m² · also corner reflector

Top: the four shapes as the radar sees them, all drawn to one scale with the size a you entered. From left to right: sphere (radius a), square flat plate (side a), triangular trihedral and square trihedral (edge a), the trihedrals seen along their axis of peak return. The bar at the top right is one wavelength at the same scale. When it would run off the panel it is cut short and marked “≫”. Under each shape is its RCS at that wavelength, in the unit named top left (dBsm). The green shape is the one you selected. Bottom: an RCS ladder in dBsm. Grey bars are the typical ranges of the reference objects in the table below, and the ones that match your value turn light. The green triangle is the value you entered, the outlined diamond the selected shape.

Shape values are the peak RCS of perfectly conducting shapes in free space, at the best aspect, in the optical region only. Real reflectors lose RCS with manufacturing error, tilt and ground reflections. The typical target values are rough microwave-band figures from Skolnik and change by 10 dB or more with aspect and frequency.

Typical radar cross sections

Typical RCS values span about 90 dB, from an insect to a ship. They are rough, microwave-band figures. A real target swings by 10 dB or more as it turns, so treat each one as a range.

Object	Typical RCS, m²	dBsm	Source
Insect (locust to fly)	10⁻⁵–10⁻⁴	−50 … −40	Skolnik ch. 2 · MIT LL
Bird (large to medium)	10⁻³–0.01	−30 … −20	Skolnik ch. 2 · MIT LL
Person	1	0	Skolnik ch. 2 · MIT LL
Bicycle	2	3	Skolnik ch. 2 · MIT LL
Fighter aircraft (small to large)	1–6	0 … 8	Skolnik ch. 2 · MIT LL
Airliner (medium to jumbo jet)	40–100	16 … 20	Skolnik ch. 2 · MIT LL
Car *	100–200	20 … 23	Skolnik ch. 2 · MIT LL
Small truck	100–200	20 … 23	Skolnik ch. 2 · MIT LL
Ship (5,000 t, L-band)	10 000	40	Skolnik ch. 2 · MIT LL
Corner reflector (30 cm to 1.5 m)	37.75–20 000	16 … 43	Skolnik ch. 2 · formula

* Skolnik’s figure for a car is 100–200 m² at microwave frequencies. Automotive radar at 77 GHz commonly works with about 10 m² (10 dBsm). That figure is engineering practice, and we found no authoritative open source for it.

RCS terms in one minute

Radar cross section (σ): The area of an ideal reflector that would send the same power back to the radar as the target does. It is not the physical size.
dBsm: Decibels relative to one square metre: 0 dBsm = 1 m², +10 dBsm = 10 m², −20 dBsm = 0.01 m².
Optical region: The object is much larger than the wavelength (ka ≳ 10). The simple shape formulas hold here.
ka: Electrical size: 2π times the size a divided by the wavelength.
Trihedral corner reflector: Three flat plates at right angles. A ray bounces off all three and leaves parallel to how it came in, straight back to the radar.

How it works

01
Convert the value

dBsm is decibels relative to 1 m²: σ_dBsm = 10·log₁₀(σ / 1 m²) and back σ = 10^σ_dBsm/10 m². 1 m² is 0 dBsm, 2 m² is 3.01 dBsm, 0.01 m² is −20 dBsm.
02
Compare with typical targets

A person is about 1 m², a bird 0.001–0.01 m², a car 100–200 m² in Skolnik’s table and a 5,000 t ship 10 000 m². Pick a target to fill in its typical value, or read the nearest match under the result.
03
Compute a simple shape

Choose the shape, its size and the radar frequency. A sphere gives π·a² at any wavelength. A flat plate gives 4π·A²/λ², a triangular trihedral 4π·a⁴/(3λ²) and a square trihedral 12π·a⁴/λ².
04
Check the optical region

The formulas hold only when the object is much larger than the wavelength. The calculator shows ka = 2π·a/λ. From 10 up it is optical, between 1 and 10 resonant (unreliable) and below 1 Rayleigh, where the RCS falls as 1/λ⁴.
05
Read what it means for range

The radar equation makes the echo power proportional to σ / R⁴. Detection range therefore grows only with σ^¼. 16 times the RCS (+12 dB) doubles the range.

Formulas

m² ↔ dBsm

σ_dBsm = 10·log₁₀(σ1 m²), σ = 10^σ_dBsm/10 m²

σ — radar cross section, m²
1 ft² = 0.09290304 m²

Sphere and flat plate (optical region)

σ_sphere = π·a², σ_plate = 4π·A²λ²

a — sphere radius, m
A — plate area, m² (square plate of side a: A = a²), radar square-on
λ = c/f — wavelength, m

Trihedral corner reflectors (peak, along the axis)

σ_tri = 4π·a⁴3·λ², σ_square = 12π·a⁴λ²

a — edge length along the open edges, m
tri — three triangular faces (common marine reflector)
square — three square faces, 9 × the triangular one

Electrical size

ka = 2π·aλ

optical ka ≥ 10, resonance 1 ≤ ka < 10, Rayleigh ka < 1 (rough guide)

Worked example

1 m², and 30 cm shapes at 10 GHz (the defaults)

conversion: 10·log₁₀(1) = 0 dBsm, and 10·log₁₀(2) = 3.01 dBsm
wavelength: λ = c / 10 GHz = 2.998 cm, so ka = 2π · 30 cm / 2.998 cm = 62.9 (optical)
sphere, radius 30 cm: σ = π · 0.3² = 0.2827 m² (−5.49 dBsm)
flat plate 30 × 30 cm: σ = 4π · 0.09² / 0.02998² = 113.3 m² (20.54 dBsm)
triangular trihedral, edge 30 cm: σ = 4π · 0.3⁴ / (3 · 0.02998²) = 37.75 m² (15.77 dBsm)
square trihedral, edge 30 cm: σ = 12π · 0.3⁴ / 0.02998² = 339.8 m² (25.31 dBsm), 9 times the triangular one

FAQ

What does dBsm mean?: dBsm means decibels relative to one square metre: σ_dBsm = 10·log₁₀(σ / 1 m²). 0 dBsm is 1 m², and every 10 dB more is ten times the area. A person is about 0 dBsm, a bird −20 to −30, a jumbo jet +20 and a 5,000 t ship +40. The log scale is practical because typical targets span about 90 dB, from an insect at −50 dBsm to a ship.
Why can a small corner reflector look as big as a boat?: Because it sends the energy straight back to the radar from a wide range of angles. Three plates at right angles bounce a ray three times and return it parallel to its path. Its RCS grows with the fourth power of its size and with frequency squared, 4π·a⁴/(3λ²). A 30 cm triangular reflector reaches 37.75 m² at 10 GHz, more than a 40–50 ft cabin cruiser at 10 m². With a 1.5 m edge it reaches about 23 600 m², more than a 5,000 t ship.
Why does RCS depend on the wavelength?: Because a flat or corner-shaped reflector focuses its echo into a beam about λ/a wide. A shorter wavelength makes that beam narrower and stronger. That is the λ² in the plate and trihedral formulas, so doubling the frequency gives 4 times the RCS (+6 dB). A sphere scatters evenly and stays at π·a² in the optical region. Objects much smaller than the wavelength fall into the Rayleigh region, where the RCS drops as 1/λ⁴. That is why insects nearly vanish for long-wavelength radars.
Why do the shape formulas only work for large objects?: Because they assume the optical region, where the object is much larger than the wavelength (ka ≳ 10). Near ka ≈ 1 the echo interferes with waves creeping round the object and the RCS oscillates. Below that it falls steeply. The calculator flags both cases. It also leaves out the dihedral and the circular trihedral, whose formulas could not be checked in a primary source.
Why does a car show 100–200 m² here when automotive radar uses about 10 m²?: Because both figures are in use and neither is wrong for its context. Skolnik’s table gives 100–200 m² for an automobile or small truck at microwave frequencies. Automotive radar at 77 GHz commonly works with about 10 m² (10 dBsm). We found no authoritative open source for that practice figure, so the table labels it as such. Either way the RCS of a real car changes by 10 dB or more with the viewing angle.

RCS tells you how strong the echo is. Whether the radar sees the target at all depends on the terrain between them. Waveshed shows where hills and buildings block the line of sight, free in your browser.

Check line of sight over terrain →

Related guides

Line of sight vs RF coverage — why a clear path matters before any radar or radio formula applies

Sources & further reading

M. I. Skolnik, Introduction to Radar Systems, 3rd ed., McGraw-Hill 2001 — ch. 2, radar cross section of targets: the typical values in the table (book, no link; table number not confirmed for a specific edition)
Introduction to Radar Systems, Lecture 4: Target Radar Cross Section — MIT Lincoln Laboratory ↗ — slide 11 examples of radar cross sections (after Skolnik), slide 8 the Rayleigh, resonance and optical regions of a sphere
E. F. Knott, J. F. Shaeffer, M. T. Tuley, Radar Cross Section, 2nd ed., Artech House 1993 — canonical shapes, including the flat plate 4πA²/λ² (p. 231) (book, no link)
G. Ecola et al., SARLink: Satellite Backscatter Connectivity using Synthetic Aperture Radar — arXiv:2402.09682 ↗ — Eq. 1, peak RCS of a square trihedral 12πd⁴/λ²
Trihedral Reflectors for Radar Applications — Millimeter Wave Products ↗ — trihedral RCS against edge length and frequency, and why the textbook value assumes free space
Radar cross section — Wikipedia ↗ — the flat-plate example and the 1.5 m corner reflector (≈ 20 000 m², after Skolnik 2nd ed.), read 7 November 2026

Radar cross section (RCS) calculator: m² ↔ dBsm

Convert m² ↔ dBsm

Simple shapes (optical region)

Typical radar cross sections

RCS terms in one minute

How it works

Convert the value

Compare with typical targets

Compute a simple shape

Check the optical region

Read what it means for range

Formulas

Worked example

FAQ

Related guides

Sources & further reading

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