ABCDEFGHIJKLMNOPQRSTUVWXYZAAABACADAEAFAGAHAI
1
2
If a cell has red text, it means: your average damage per attack (DPA) with GWM is less than your average DPA without.
If a cell is highlighted in red, it means: you can't hit at all with GWM.

Note: The "Hit On" column is the value you would need to roll on a d20, before modifiers, to hit without GWM.
(e.g. if your bonus is +5 and you're attacking someone with AC 12, you hit on a 7.)
When using GWM, your actual roll needed to hit is 5 higher.

Additional note: A typical two-handed weapon, like a greatsword, will deal somewhere between 2d6+2 and 2d6+8, depending on your Str and enchantments.
If we assume a Str of 20 and no enchants, average damage is 12.

So if you're average and hitting on a 9+ or better (or 11+ or better and have advantage), use your GWM. (Unless you can already kill the target in one hit.)
The relevant cell is outlined in sky blue.
If you want to see more clearly how these numbers apply to you, make a copy of this spreadsheet and enter your attack bonus and average damage in the cells below.

Your column is the one that's bold and underlined.
3
4
5
6
7
8
9
10
Attack bonus
+5
11
Average damage
10
12
13
DPA increase from Great Weapon Master, without advantage
14
Average damage
15
ACHit On12345678910111213141516171819202122232425262728293031
16
72+6.756.506.256.005.755.505.255.004.754.504.254.003.753.503.253.002.752.502.252.001.751.501.251.000.750.500.250.00-0.25-0.50-0.75
17
83+6.256.005.755.505.255.004.754.504.254.003.753.503.253.002.752.502.252.001.751.501.251.000.750.500.250.00-0.25-0.50-0.75-1.00-1.25
18
94+5.755.505.255.004.754.504.254.003.753.503.253.002.752.502.252.001.751.501.251.000.750.500.250.00-0.25-0.50-0.75-1.00-1.25-1.50-1.75
19
105+5.255.004.754.504.254.003.753.503.253.002.752.502.252.001.751.501.251.000.750.500.250.00-0.25-0.50-0.75-1.00-1.25-1.50-1.75-2.00-2.25
20
116+4.754.504.254.003.753.503.253.002.752.502.252.001.751.501.251.000.750.500.250.00-0.25-0.50-0.75-1.00-1.25-1.50-1.75-2.00-2.25-2.50-2.75
21
127+4.254.003.753.503.253.002.752.502.252.001.751.501.251.000.750.500.250.00-0.25-0.50-0.75-1.00-1.25-1.50-1.75-2.00-2.25-2.50-2.75-3.00-3.25
22
138+3.753.503.253.002.752.502.252.001.751.501.251.000.750.500.250.00-0.25-0.50-0.75-1.00-1.25-1.50-1.75-2.00-2.25-2.50-2.75-3.00-3.25-3.50-3.75
23
149+3.253.002.752.502.252.001.751.501.251.000.750.500.250.00-0.25-0.50-0.75-1.00-1.25-1.50-1.75-2.00-2.25-2.50-2.75-3.00-3.25-3.50-3.75-4.00-4.25
24
1510+2.752.502.252.001.751.501.251.000.750.500.250.00-0.25-0.50-0.75-1.00-1.25-1.50-1.75-2.00-2.25-2.50-2.75-3.00-3.25-3.50-3.75-4.00-4.25-4.50-4.75
25
1611+2.252.001.751.501.251.000.750.500.250.00-0.25-0.50-0.75-1.00-1.25-1.50-1.75-2.00-2.25-2.50-2.75-3.00-3.25-3.50-3.75-4.00-4.25-4.50-4.75-5.00-5.25
26
1712+1.751.501.251.000.750.500.250.00-0.25-0.50-0.75-1.00-1.25-1.50-1.75-2.00-2.25-2.50-2.75-3.00-3.25-3.50-3.75-4.00-4.25-4.50-4.75-5.00-5.25-5.50-5.75
27
1813+1.251.000.750.500.250.00-0.25-0.50-0.75-1.00-1.25-1.50-1.75-2.00-2.25-2.50-2.75-3.00-3.25-3.50-3.75-4.00-4.25-4.50-4.75-5.00-5.25-5.50-5.75-6.00-6.25
28
1914+0.750.500.250.00-0.25-0.50-0.75-1.00-1.25-1.50-1.75-2.00-2.25-2.50-2.75-3.00-3.25-3.50-3.75-4.00-4.25-4.50-4.75-5.00-5.25-5.50-5.75-6.00-6.25-6.50-6.75
29
2015+0.250.00-0.25-0.50-0.75-1.00-1.25-1.50-1.75-2.00-2.25-2.50-2.75-3.00-3.25-3.50-3.75-4.00-4.25-4.50-4.75-5.00-5.25-5.50-5.75-6.00-6.25-6.50-6.75-7.00-7.25
30
2116+-0.25-0.50-0.75-1.00-1.25-1.50-1.75-2.00-2.25-2.50-2.75-3.00-3.25-3.50-3.75-4.00-4.25-4.50-4.75-5.00-5.25-5.50-5.75-6.00-6.25-6.50-6.75-7.00-7.25-7.50-7.75
31
2217+-0.75-1.00-1.25-1.50-1.75-2.00-2.25-2.50-2.75-3.00-3.25-3.50-3.75-4.00-4.25-4.50-4.75-5.00-5.25-5.50-5.75-6.00-6.25-6.50-6.75-7.00-7.25-7.50-7.75-8.00-8.25
32
2318+-1.25-1.50-1.75-2.00-2.25-2.50-2.75-3.00-3.25-3.50-3.75-4.00-4.25-4.50-4.75-5.00-5.25-5.50-5.75-6.00-6.25-6.50-6.75-7.00-7.25-7.50-7.75-8.00-8.25-8.50-8.75
33
2419+-1.75-2.00-2.25-2.50-2.75-3.00-3.25-3.50-3.75-4.00-4.25-4.50-4.75-5.00-5.25-5.50-5.75-6.00-6.25-6.50-6.75-7.00-7.25-7.50-7.75-8.00-8.25-8.50-8.75-9.00-9.25
34
2520+-2.25-2.50-2.75-3.00-3.25-3.50-3.75-4.00-4.25-4.50-4.75-5.00-5.25-5.50-5.75-6.00-6.25-6.50-6.75-7.00-7.25-7.50-7.75-8.00-8.25-8.50-8.75-9.00-9.25-9.50-9.75
35
36
DPA increase from Great Weapon Master, with advantage
37
Average damage
38
ACHit On12345678910111213141516171819202122232425262728293031
39
72+9.018.938.848.758.668.588.498.408.318.238.148.057.967.887.797.707.617.537.447.357.267.187.097.006.916.836.746.656.566.486.39
40
83+8.668.558.448.338.218.107.997.887.767.657.547.437.317.207.096.986.866.756.646.536.416.306.196.085.965.855.745.635.515.405.29
41
94+8.268.137.997.857.717.587.447.307.167.036.896.756.616.486.346.206.065.935.795.655.515.385.245.104.964.834.694.554.414.284.14
42
105+7.817.657.497.337.167.006.846.686.516.356.196.035.865.705.545.385.215.054.894.734.564.404.244.083.913.753.593.433.263.102.94
43
116+7.317.136.946.756.566.386.196.005.815.635.445.255.064.884.694.504.314.133.943.753.563.383.193.002.812.632.442.252.061.881.69
44
127+6.766.556.346.135.915.705.495.285.064.854.644.434.214.003.793.583.363.152.942.732.512.302.091.881.661.451.241.030.810.600.39
45
138+6.165.935.695.455.214.984.744.504.264.033.793.553.313.082.842.602.362.131.891.651.411.180.940.700.460.22-0.01-0.25-0.49-0.72-0.96
46
149+5.515.254.994.734.464.203.943.683.413.152.892.632.362.101.841.581.311.050.790.520.260.00-0.26-0.52-0.79-1.05-1.31-1.58-1.84-2.10-2.36
47
1510+4.814.534.243.953.663.383.092.802.512.231.941.651.361.080.790.500.21-0.08-0.36-0.65-0.94-1.23-1.51-1.80-2.09-2.38-2.66-2.95-3.24-3.53-3.81
48
1611+4.063.753.443.132.812.502.191.881.561.250.940.630.310.00-0.31-0.63-0.94-1.25-1.56-1.88-2.19-2.50-2.81-3.13-3.44-3.75-4.06-4.38-4.69-5.00-5.31
49
1712+3.262.932.592.251.911.581.240.900.560.23-0.11-0.45-0.79-1.13-1.46-1.80-2.14-2.48-2.81-3.15-3.49-3.83-4.16-4.50-4.84-5.18-5.51-5.85-6.19-6.53-6.86
50
1813+2.412.051.691.330.960.600.24-0.13-0.49-0.85-1.21-1.58-1.94-2.30-2.66-3.03-3.39-3.75-4.11-4.48-4.84-5.20-5.56-5.93-6.29-6.65-7.01-7.38-7.74-8.10-8.46
51
1914+1.511.130.740.35-0.04-0.43-0.81-1.20-1.59-1.98-2.36-2.75-3.14-3.53-3.91-4.30-4.69-5.08-5.46-5.85-6.24-6.63-7.01-7.40-7.79-8.18-8.56-8.95-9.34-9.73-10.11
52
2015+0.560.15-0.26-0.68-1.09-1.50-1.91-2.33-2.74-3.15-3.56-3.98-4.39-4.80-5.21-5.63-6.04-6.45-6.86-7.28-7.69-8.10-8.51-8.93-9.34-9.75-10.16-10.58-10.99-11.40-11.81
53
2116+-0.44-0.88-1.31-1.75-2.19-2.63-3.06-3.50-3.94-4.38-4.81-5.25-5.69-6.13-6.56-7.00-7.44-7.88-8.31-8.75-9.19-9.63-10.06-10.50-10.94-11.38-11.81-12.25-12.69-13.13-13.56
54
2217+-1.49-1.95-2.41-2.88-3.34-3.80-4.26-4.73-5.19-5.65-6.11-6.58-7.04-7.50-7.96-8.43-8.89-9.35-9.81-10.28-10.74-11.20-11.66-12.13-12.59-13.05-13.51-13.98-14.44-14.90-15.36
55
2318+-2.59-3.08-3.56-4.05-4.54-5.03-5.51-6.00-6.49-6.98-7.46-7.95-8.44-8.93-9.41-9.90-10.39-10.88-11.36-11.85-12.34-12.83-13.31-13.80-14.29-14.78-15.26-15.75-16.24-16.73-17.21
56
2419+-3.74-4.25-4.76-5.28-5.79-6.30-6.81-7.33-7.84-8.35-8.86-9.38-9.89-10.40-10.91-11.43-11.94-12.45-12.96-13.48-13.99-14.50-15.01-15.53-16.04-16.55-17.06-17.58-18.09-18.60-19.11
57
2520+-4.94-5.48-6.01-6.55-7.09-7.63-8.16-8.70-9.24-9.78-10.31-10.85-11.39-11.93-12.46-13.00-13.54-14.08-14.61-15.15-15.69-16.23-16.76-17.30-17.84-18.38-18.91-19.45-19.99-20.53-21.06
58
59
DPA increase from Great Weapon Master, with Elven Accuracy
60
Average damage
61
ACHit On12345678910111213141516171819202122232425262728293031
62
72+9.709.689.659.629.609.579.549.529.499.469.439.419.389.359.339.309.279.259.229.199.179.149.119.099.069.039.008.988.958.928.90
63
83+9.539.499.459.409.369.329.289.249.199.159.119.079.038.998.948.908.868.828.788.738.698.658.618.578.528.488.448.408.368.328.27
64
94+9.309.249.189.129.069.008.948.888.818.758.698.638.578.518.458.398.338.278.218.158.098.037.977.917.847.787.727.667.607.547.48
65
105+9.018.928.848.768.678.598.518.428.348.268.178.098.017.937.847.767.687.597.517.437.347.267.187.097.016.936.846.766.686.606.51
66
116+8.648.538.428.318.208.097.987.887.777.667.557.447.337.227.117.006.896.786.676.566.456.346.236.136.025.915.805.695.585.475.36
67
127+8.208.067.927.787.647.507.367.227.086.946.806.666.526.396.256.115.975.835.695.555.415.275.134.994.854.714.574.434.294.164.02
68
138+7.677.497.327.156.976.806.636.466.286.115.945.765.595.425.245.074.904.724.554.384.204.033.863.693.513.343.172.992.822.652.47
69
149+7.046.836.626.416.205.995.785.575.365.154.944.734.524.314.093.883.673.463.253.042.832.622.412.201.991.781.571.361.150.940.72
70
1510+6.326.075.815.565.315.064.814.564.304.053.803.553.303.042.792.542.292.041.781.531.281.030.780.530.270.02-0.23-0.48-0.73-0.99-1.24
71
1611+5.485.194.894.594.304.003.703.413.112.812.522.221.921.631.331.030.730.440.14-0.16-0.45-0.75-1.05-1.34-1.64-1.94-2.23-2.53-2.83-3.13-3.42
72
1712+4.534.193.843.503.152.812.462.121.771.421.080.730.390.04-0.30-0.65-1.00-1.34-1.69-2.03-2.38-2.72-3.07-3.42-3.76-4.11-4.45-4.80-5.14-5.49-5.83
73
1813+3.463.062.662.271.871.471.070.670.28-0.12-0.52-0.92-1.32-1.72-2.11-2.51-2.91-3.31-3.71-4.10-4.50-4.90-5.30-5.70-6.09-6.49-6.89-7.29-7.69-8.09-8.48
74
1914+2.261.801.350.890.44-0.02-0.47-0.93-1.38-1.83-2.29-2.74-3.20-3.65-4.11-4.56-5.01-5.47-5.92-6.38-6.83-7.29-7.74-8.20-8.65-9.10-9.56-10.01-10.47-10.92-11.38
75
2015+0.910.40-0.12-0.63-1.15-1.66-2.17-2.69-3.20-3.72-4.23-4.75-5.26-5.78-6.29-6.80-7.32-7.83-8.35-8.86-9.38-9.89-10.40-10.92-11.43-11.95-12.46-12.98-13.49-14.01-14.52
76
2116+-0.58-1.16-1.73-2.31-2.89-3.47-4.05-4.63-5.20-5.78-6.36-6.94-7.52-8.09-8.67-9.25-9.83-10.41-10.98-11.56-12.14-12.72-13.30-13.88-14.45-15.03-15.61-16.19-16.77-17.34-17.92
77
2217+-2.22-2.87-3.51-4.16-4.80-5.45-6.10-6.74-7.39-8.03-8.68-9.32-9.97-10.62-11.26-11.91-12.55-13.20-13.84-14.49-15.13-15.78-16.43-17.07-17.72-18.36-19.01-19.65-20.30-20.95-21.59
78
2318+-4.03-4.74-5.46-6.18-6.89-7.61-8.33-9.05-9.76-10.48-11.20-11.91-12.63-13.35-14.06-14.78-15.50-16.21-16.93-17.65-18.36-19.08-19.80-20.52-21.23-21.95-22.67-23.38-24.10-24.82-25.53
79
2419+-6.00-6.79-7.58-8.38-9.17-9.96-10.75-11.54-12.34-13.13-13.92-14.71-15.50-16.30-17.09-17.88-18.67-19.46-20.25-21.05-21.84-22.63-23.42-24.21-25.01-25.80-26.59-27.38-28.17-28.97-29.76
80
2520+-8.15-9.02-9.89-10.76-11.63-12.50-13.37-14.25-15.12-15.99-16.86-17.73-18.60-19.47-20.34-21.21-22.08-22.95-23.82-24.69-25.56-26.43-27.30-28.18-29.05-29.92-30.79-31.66-32.53-33.40-34.27
81