I shouldn't do this, but I noticed a flaw with the original calc from this blog. I'll use this method, but there's a problem. Captain said that the trees in Tenrou are massive, and while that's true for some of them, not every tree is that big so instead of assuming every tree is 20 meters tall (despite being a lowball considering how big that tree actually was) I'll use the average since most of the trees seems to be around there. In fact, this appears to be the case for some zones of the island where the trees' trunks are really wide, while the other trees seems to be normal.
I'm also not sure about the island's size but considering I have no idea how to get it scaled I'll go with the currently accepted size at the moment.
Zancrow's Bellow[]
Tree height = 15px = 7 meters (trees are usually around 5 and 7 meters but I'll use 7 meters instead since some of them are fairly high).
Crater diameter 1 = 124px = 57.86m.
Crater diameter 2 = 57.86/2 = 28.93m.
Crater depth = 58px = 27.06m.
This looks like a spherical cap, and most likely is, given the shape shown here. Still though, I’m unsure which shape fits the most.
R = (h^2 + r1 * r2)/2h = (27.06 ^2 + (57.86 /2) * (28.93 /2))/(2*27.06) = 21.26230 meters
V = 1/3 * pi * h^2 *(3R - h) = 1/3 * pi * 27.06^2 * (3 * 21.26230 - 27.06) = 28162.323 m^3
Energy
Pulverization (accepted): 28162.323 * 1000000 * 214.35 = 6036593932005 joules or 1.44 Kilotons (Small Town level)
Vaporization: 28162.323 * 1000000 * 25700 = 723771700734967 joules or 172 Kilotons (Large Town level)
Natsu's Roar[]
This one relies on the same assumption.
Tree height = 70px = 7 meters
Ratio = 0.1 m/px.
Beam size = 220px = 22m.
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Beam size = 7px = 22m
Ratio = 3.1428571428571429 m/px
Crater diameter = 204.2857m.
Cylinder: https://www.calculator.net/volume-calculator.html
V = 187410869.60963 meters3
According to Torch the blast should have 2/3 of its total volume considering it at least destroyed that much.
187310869.60963*2/3 = 124873913.073
Pulverization: 124873913.073 * 1000000 * 214.35 = 26766723267216127 joules or 6.39 Megatons (City level)
Conclusion[]
Zancrow's Belly: 1.44 Kilotons Small Town level
Natsu's Roar: 6.39 Megatons City level