The dBm sometimes called dBmW or decibel-milliwatts is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW) (see Conversion Table below). It is used in RF and optical fiber as a convenient measure of absolute power because of its ability to express very large or very small values in a short form. By comparison, the decibel (dB) is a dimensionless unit used to quantify the relationship between two values, such as signal-to-noise ratio. In audio and telephony dBm is typically referenced relative to an impedance of 600 ohms, whereas in the RF dBm is typically referenced relative to an impedance of 50 ohms.
What is the difference between " dB " , " dBi " and " dBm "
The DB , dBi and dBm are not synonymous , they mean very different things .
What dB
The dB ( decibel ) is a relative measure two different power levels , is used to declare the gain or loss of a device relative to another device. That is, the dB is a logarithmic unit used to express the ratio of two values of a physical quantity , often a power or intensity . This is the basis of dBx .
What is dBi
The dBi ( dB isotropic ) - Shows the gain of an antenna relative to a hypothetical isotropic antenna , which uniformly distributes energy in all directions ( ground plane ) .
What is dBm
The dBm ( dB mW ) is the power relative to 1 milliwatt widely used in RF , audio and telephony ( and we see this article) .
Examples of Extremes dBm
-120.0 DBm signal strength received on the ground from a GPS satellite
+ 80 dBm Power 100 kW transmission of a radio commercial
A convenient way to measure the dimensionless power is :
The exact formula : P ( dBm ) = 10 · log10 ( P ( W ) / 1 mW )
where
P ( dBm ) = Power expressed in dBm
P ( W ) = absolute power measured in Watts
mW = milliwatts
log10 = log base 10
From this formula , the power in dBm of 1 Watt is 30 dBm . Because the calculation is logarithmic , each increase of 3dBm is roughly equivalent to twice the actual power of a signal .
In the table below :
Conversion Table dBm Watts x (50 Ohms) x Volts (RMS)
Rel power.
@ 50 Ohms
|
Abs power
|
Voltage (RMS)
|
Rel power.
@ 50 Ohms
|
Power Abs.
|
Voltage (RMS)
|
|
-150 dBm
|
1.00 aW
|
7.08 nV
|
-50 dBm
|
10.00 nW
|
708.35 uV
|
|
-149 dBm
|
1.26 aW
|
7.95 nV
|
-49 dBm
|
12.59 nW
|
794.78 uV
|
|
-148 dBm
|
1.58 aW
|
8.92 nV
|
-48 dBm
|
15.85 nW
|
891.76 uV
|
|
-147 dBm
|
2.00 aW
|
10.01 nV
|
-47 dBm
|
19.95 nW
|
1.00 mV
|
|
-146 dBm
|
2.51 aW
|
11.23 nV
|
-46 dBm
|
25.12 nW
|
1.12 mV
|
|
-145 dBm
|
3.16 aW
|
12.60 nV
|
-45 dBm
|
31.62 nW
|
1.26 mV
|
|
-144 dBm
|
3.98 aW
|
14.13 nV
|
-44 dBm
|
39.81 nW
|
1.41 mV
|
|
-143 dBm
|
5.01 aW
|
15.86 nV
|
-43 dBm
|
50.12 nW
|
1.59 mV
|
|
-142 dBm
|
6.31 aW
|
17.79 nV
|
-42 dBm
|
63.10 nW
|
1.78 mV
|
|
-141 dBm
|
7.94 aW
|
19.96 nV
|
-41 dBm
|
79.43 nW
|
2.00 mV
|
|
-140 dBm
|
10.00 aW
|
22.40 nV
|
-40 dBm
|
100.00 nW
|
2.24 mV
|
|
-139 dBm
|
12.59 aW
|
25.13 nV
|
-39 dBm
|
125.89 nW
|
2.51 mV
|
|
-138 dBm
|
15.85 aW
|
28.20 nV
|
-38 dBm
|
158.49 nW
|
2.82 mV
|
|
-137 dBm
|
19.95 aW
|
31.64 nV
|
-37 dBm
|
199.53 nW
|
3.16 mV
|
|
-136 dBm
|
25.12 aW
|
35.50 nV
|
-36 dBm
|
251.19 nW
|
3.55 mV
|
|
-135 dBm
|
31.62 aW
|
39.83 nV
|
-35 dBm
|
316.23 nW
|
3.98 mV
|
|
-134 dBm
|
39.81 aW
|
44.69 nV
|
-34 dBm
|
398.11 nW
|
4.47 mV
|
|
-133 dBm
|
50.12 aW
|
50.15 nV
|
-33 dBm
|
501.19 nW
|
5.01 mV
|
|
-132 dBm
|
63.10 aW
|
56.27 nV
|
-32 dBm
|
630.96 nW
|
5.63 mV
|
|
-131 dBm
|
79.43 aW
|
63.13 nV
|
-31 dBm
|
794.33 nW
|
6.31 mV
|
|
-130 dBm
|
100.00 aW
|
70.84 nV
|
-30 dBm
|
1.00 uW
|
7.08 mV
|
|
-129 dBm
|
125.89 aW
|
79.48 nV
|
-29 dBm
|
1.26 uW
|
7.95 mV
|
|
-128 dBm
|
158.49 aW
|
89.18 nV
|
-28 dBm
|
1.58 uW
|
8.92 mV
|
|
-127 dBm
|
199.53 aW
|
100.06 nV
|
-27 dBm
|
2.00 uW
|
10.01 mV
|
|
-126 dBm
|
251.19 aW
|
112.27 nV
|
-26 dBm
|
2.51 uW
|
11.23 mV
|
|
-125 dBm
|
316.23 aW
|
125.96 nV
|
-25 dBm
|
3.16 uW
|
12.60 mV
|
|
-124 dBm
|
398.11 aW
|
141.33 nV
|
-24 dBm
|
3.98 uW
|
14.13 mV
|
|
-123 dBm
|
501.19 aW
|
158.58 nV
|
-23 dBm
|
5.01 uW
|
15.86 mV
|
|
-122 dBm
|
630.96 aW
|
177.93 nV
|
-22 dBm
|
6.31 uW
|
17.79 mV
|
|
-121 dBm
|
794.33 aW
|
199.64 nV
|
-21 dBm
|
7.94 uW
|
19.96 mV
|
|
-120 dBm
|
1.00 fW
|
224.00 nV
|
-20 dBm
|
10.00 uW
|
22.40 mV
|
|
-119 dBm
|
1.26 fW
|
251.33 nV
|
-19 dBm
|
12.59 uW
|
25.13 mV
|
|
-118 dBm
|
1.58 fW
|
282.00 nV
|
-18 dBm
|
15.85 uW
|
28.20 mV
|
|
-117 dBm
|
2.00 fW
|
316.41 nV
|
-17 dBm
|
19.95 uW
|
31.64 mV
|
|
-116 dBm
|
2.51 fW
|
355.02 nV
|
-16 dBm
|
25.12 uW
|
35.50 mV
|
|
-115 dBm
|
3.16 fW
|
398.33 nV
|
-15 dBm
|
31.62 uW
|
39.83 mV
|
|
-114 dBm
|
3.98 fW
|
446.94 nV
|
-14 dBm
|
39.81 uW
|
44.69 mV
|
|
-113 dBm
|
5.01 fW
|
501.47 nV
|
-13 dBm
|
50.12 uW
|
50.15 mV
|
|
-112 dBm
|
6.31 fW
|
562.66 nV
|
-12 dBm
|
63.10 uW
|
56.27 mV
|
|
-111 dBm
|
7.94 fW
|
631.32 nV
|
-11 dBm
|
79.43 uW
|
63.13 mV
|
|
-110 dBm
|
10.00 fW
|
708.35 nV
|
-10 dBm
|
100.00 uW
|
70.84 mV
|
|
-109 dBm
|
12.59 fW
|
794.78 nV
|
-9 dBm
|
125.89 uW
|
79.48 mV
|
|
-108 dBm
|
15.85 fW
|
891.76 nV
|
-8 dBm
|
158.49 uW
|
89.18 mV
|
|
-107 dBm
|
19.95 fW
|
1.00 uV
|
-7 dBm
|
199.53 uW
|
100.06 mV
|
|
-106 dBm
|
25.12 fW
|
1.12 uV
|
-6 dBm
|
251.19 uW
|
112.27 mV
|
|
-105 dBm
|
31.62 fW
|
1.26 uV
|
-5 dBm
|
316.23 uW
|
125.96 mV
|
|
-104 dBm
|
39.81 fW
|
1.41 uV
|
-4 dBm
|
398.11 uW
|
141.33 mV
|
|
-103 dBm
|
50.12 fW
|
1.59 uV
|
-3 dBm
|
501.19 uW
|
158.58 mV
|
|
-102 dBm
|
63.10 fW
|
1.78 uV
|
-2 dBm
|
630.96 uW
|
177.93 mV
|
|
-101 dBm
|
79.43 fW
|
2.00 uV
|
-1 dBm
|
794.33 uW
|
199.64 mV
|
|
-100 dBm
|
100.00 fW
|
2.24 uV
|
+0 dBm
|
1.00 mW
|
224.00 mV
|
|
-99 dBm
|
125.89 fW
|
2.51 uV
|
+1 dBm
|
1.26 mW
|
251.33 mV
|
|
-98 dBm
|
158.49 fW
|
2.82 uV
|
+2 dBm
|
1.58 mW
|
282.00 mV
|
|
-97 dBm
|
199.53 fW
|
3.16 uV
|
+3 dBm
|
2.00 mW
|
316.41 mV
|
|
-96 dBm
|
251.19 fW
|
3.55 uV
|
+4 dBm
|
2.51 mW
|
355.02 mV
|
|
-95 dBm
|
316.23 fW
|
3.98 uV
|
+5 dBm
|
3.16 mW
|
398.33 mV
|
|
-94 dBm
|
398.11 fW
|
4.47 uV
|
+6 dBm
|
3.98 mW
|
446.94 mV
|
|
-93 dBm
|
501.19 fW
|
5.01 uV
|
+7 dBm
|
5.01 mW
|
501.47 mV
|
|
-92 dBm
|
630.96 fW
|
5.63 uV
|
+8 dBm
|
6.31 mW
|
562.66 mV
|
|
-91 dBm
|
794.33 fW
|
6.31 uV
|
+9 dBm
|
7.94 mW
|
631.32 mV
|
|
-90 dBm
|
1.00 pW
|
7.08 uV
|
+10 dBm
|
10.00 mW
|
708.35 mV
|
|
-89 dBm
|
1.26 pW
|
7.95 uV
|
+11 dBm
|
12.59 mW
|
794.78 mV
|
|
-88 dBm
|
1.58 pW
|
8.92 uV
|
+12 dBm
|
15.85 mW
|
891.76 mV
|
|
-87 dBm
|
2.00 pW
|
10.01 uV
|
+13 dBm
|
19.95 mW
|
1.00 V
|
|
-86 dBm
|
2.51 pW
|
11.23 uV
|
+14 dBm
|
25.12 mW
|
1.12 V
|
|
-85 dBm
|
3.16 pW
|
12.60 uV
|
+15 dBm
|
31.62 mW
|
1.26 V
|
|
-84 dBm
|
3.98 pW
|
14.13 uV
|
+16 dBm
|
39.81 mW
|
1.41 V
|
|
-83 dBm
|
5.01 pW
|
15.86 uV
|
+17 dBm
|
50.12 mW
|
1.59 V
|
|
-82 dBm
|
6.31 pW
|
17.79 uV
|
+18 dBm
|
63.10 mW
|
1.78 V
|
|
-81 dBm
|
7.94 pW
|
19.96 uV
|
+19 dBm
|
79.43 mW
|
2.00 V
|
|
-80 dBm
|
10.00 pW
|
22.40 uV
|
+20 dBm
|
100.00 mW
|
2.24 V
|
|
-79 dBm
|
12.59 pW
|
25.13 uV
|
+21 dBm
|
125.89 mW
|
2.51 V
|
|
-78 dBm
|
15.85 pW
|
28.20 uV
|
+22 dBm
|
158.49 mW
|
2.82 V
|
|
-77 dBm
|
19.95 pW
|
31.64 uV
|
+23 dBm
|
199.53 mW
|
3.16 V
|
|
-76 dBm
|
25.12 pW
|
35.50 uV
|
+24 dBm
|
251.19 mW
|
3.55 V
|
|
-75 dBm
|
31.62 pW
|
39.83 uV
|
+25 dBm
|
316.23 mW
|
3.98 V
|
|
-74 dBm
|
39.81 pW
|
44.69 uV
|
+26 dBm
|
398.11 mW
|
4.47 V
|
|
-73 dBm
|
50.12 pW
|
50.15 uV
|
+27 dBm
|
501.19 mW
|
5.01 V
|
|
-72 dBm
|
63.10 pW
|
56.27 uV
|
+28 dBm
|
630.96 mW
|
5.63 V
|
|
-71 dBm
|
79.43 pW
|
63.13 uV
|
+29 dBm
|
794.33 mW
|
6.31 V
|
|
-70 dBm
|
100.00 pW
|
70.84 uV
|
+30 dBm
|
1.00 W
|
7.08 V
|
|
-69 dBm
|
125.89 pW
|
79.48 uV
|
+31 dBm
|
1.26 W
|
7.95 V
|
|
-68 dBm
|
158.49 pW
|
89.18 uV
|
+32 dBm
|
1.58 W
|
8.92 V
|
|
-67 dBm
|
199.53 pW
|
100.06 uV
|
+33 dBm
|
2.00 W
|
10.01 V
|
|
-66 dBm
|
251.19 pW
|
112.27 uV
|
+34 dBm
|
2.51 W
|
11.23 V
|
|
-65 dBm
|
316.23 pW
|
125.96 uV
|
+35 dBm
|
3.16 W
|
12.60 V
|
|
-64 dBm
|
398.11 pW
|
141.33 uV
|
+36 dBm
|
3.98 W
|
14.13 V
|
|
-63 dBm
|
501.19 pW
|
158.58 uV
|
+37 dBm
|
5.01 W
|
15.86 V
|
|
-62 dBm
|
630.96 pW
|
177.93 uV
|
+38 dBm
|
6.31 W
|
17.79 V
|
|
-61 dBm
|
794.33 pW
|
199.64 uV
|
+39 dBm
|
7.94 W
|
19.96 V
|
|
-60 dBm
|
1.00 nW
|
224.00 uV
|
+40 dBm
|
10.00 W
|
22.40 V
|
|
-59 dBm
|
1.26 nW
|
251.33 uV
|
+41 dBm
|
12.59 W
|
25.13 V
|
|
-58 dBm
|
1.58 nW
|
282.00 uV
|
+42 dBm
|
15.85 W
|
28.20 V
|
|
-57 dBm
|
2.00 nW
|
316.41 uV
|
+43 dBm
|
19.95 W
|
31.64 V
|
|
-56 dBm
|
2.51 nW
|
355.02 uV
|
+44 dBm
|
25.12 W
|
35.50 V
|
|
-55 dBm
|
3.16 nW
|
398.33 uV
|
+45 dBm
|
31.62 W
|
39.83 V
|
|
-54 dBm
|
3.98 nW
|
446.94 uV
|
+46 dBm
|
39.81 W
|
44.69 V
|
|
-53 dBm
|
5.01 nW
|
501.47 uV
|
+47 dBm
|
50.12 W
|
50.15 V
|
|
-52 dBm
|
6.31 nW
|
562.66 uV
|
+48 dBm
|
63.10 W
|
56.27 V
|
|
-51 dBm
|
7.94 nW
|
631.32 uV
|
+49 dBm
|
79.43 W
|
63.13 V
|
|
|
|
+50 dBm
|
100.00 W
|
70.84 V
|
