Why Every Explosive Is Measured Against a Gram of TNT
Blast work does not quote explosives in their own energy content. It quotes them in TNT equivalent, because one number then covers a detonator, a borehole charge, an industrial accident and a nuclear yield. The anchor of that system is a single defined figure: one gram of TNT is taken to be 4 184 joules. Everything above it — the kilogram, the tonne, the kiloton — is that figure multiplied by a thousand, and every charge of some other explosive is first rescaled into an equivalent mass of TNT before its energy is worked out. The joule-to-megajoule step is where those charge masses stop being grams and start being a yield you can put in a report.
Four Things the Number Assumes Before You Convert It
The Gram of TNT Was Defined, Not Weighed
Relative Effectiveness Rescales the Mass
A Kiloton Counts Energy, Not Explosive
Blast Radius Follows the Cube Root
Sizing a Charge From Its Relative Effectiveness
The order matters. Correct the real explosive into TNT equivalent, express that as grams, and only then let the conversion carry it up to megajoules — doing it the other way round buries the effectiveness factor where nobody can audit it.
Convert the charge to TNT equivalent
Take the actual mass and multiply by the relative effectiveness factor for that product. A 25 kg ANFO borehole charge at 0.74 becomes 18.5 kg TNT equivalent; a 567 g C-4 demolition block at 1.34 becomes 759.78 g.
Turn that mass into joules at 4 184 per gram
18.5 kg is 18 500 g, so 18 500 × 4 184 = 77 404 000 J. The block gives 759.78 × 4 184 = 3 178 919.52 J. Type either figure into the joule field and the megajoule side answers on the same keystroke.
Turn the pair around when the yield is the given
Incident reports usually publish a yield, not a charge. Press the swap arrows to lead with megajoules, enter the published figure, and read the joules back — divide by 4 184 and you have the equivalent grams of TNT that yield implies.
Lift the digits into the scaled-distance sheet
The copy control above each field hands back the number on its own, with no unit and no thousands spacing, so it drops straight into the cell that will take a cube root. Ctrl+C inside a field does the same thing.
Charges and Blast Events by TNT Equivalent
Every row starts from a real charge, applies the relative effectiveness factor where the material is not TNT, and converts the resulting equivalent mass at 4 184 J per gram. Joules are shown while they stay readable; above a tonne only the megajoule column is worth printing.
| Charge or event | TNT equivalent | Energy (J) | Energy (MJ) |
|---|---|---|---|
| Detonator base charge, 0.8 g PETN | 1.328 g | 5 556.352 | 0.005556352 |
| Signal charge, 10 g black powder | 5.5 g | 23 012 | 0.023012 |
| Demolition block, 567 g C-4 | 759.78 g | 3 178 919.52 | 3.17891952 |
| 1 kg TNT reference charge | 1 000 g | 4 184 000 | 4.184 |
| 1 kg PETN booster | 1 660 g | 6 945 440 | 6.94544 |
| 25 kg ANFO borehole charge | 18.5 kg | 77 404 000 | 77.404 |
| 1 tonne TNT | 1 t | 4.184e+9 | 4 184 |
| One kiloton of TNT equivalent | 1 000 t | 4.184e+12 | 4 184 000 |
The spread is the point. A detonator and a kiloton sit nearly nine orders of magnitude apart, yet both are described by the same defined gram, which is exactly why the convention has outlasted every attempt to replace it. Notice also the two one-kilogram rows: PETN carries two-thirds more energy than TNT for the same weight, so a charge specified by mass and a charge specified by yield are never the same specification.
What This Pair of Fields Adds to a Yield Calculation
Retry a Charge Mass Without Clearing the Field
Effectiveness factors get argued over, so the equivalent mass moves. Editing the joule figure re-answers the megajoule side as each digit lands, which makes a range of assumptions quick to walk through.
Detonator-Sized Values Stay Off Zero
A few thousand joules is a five-decimal megajoule figure. Output carries eight decimals before it switches to exponent form, so a cap or a squib never rounds away to nothing on the way up the scale.
Bare Yield Digits for the Blast Model
Copied values arrive without a unit suffix or spaced thousands, which matters when the destination is a scaled-distance formula that will reject anything but a plain number.
Set a Yield Beside toe and tce
The searchable list on either side reaches tonne of oil equivalent and tonne of coal equivalent, so a blast figure can be read against the energy units a non-specialist audience already recognises.
Blast and Yield Questions Behind the TNT Equivalent
Why is a gram of TNT fixed at exactly 4 184 joules?
Because it was defined into existence rather than discovered. The equivalent was set at 1 000 thermochemical calories per gram, and the thermochemical calorie is itself defined as exactly 4.184 J, so the product is exactly 4 184 J — that is 0.004184 MJ. The shared digits with the kilocalorie are not a coincidence and not a physical fact about trinitrotoluene; they are the same definition seen twice. The payoff is that yields stated decades apart, by different agencies, remain directly comparable.
What does a relative effectiveness factor do to a charge calculation?
It converts real mass into equivalent mass before any energy arithmetic happens. A kilogram of ANFO at 0.74 counts as 740 g of TNT, giving 3 096 160 J or 3.09616 MJ; a kilogram of PETN at 1.66 counts as 1 660 g, giving 6 945 440 J or 6.94544 MJ. That is a ratio of 2.24 between two charges a scale would call identical. Note also that factors quoted for air-blast pressure, for impulse and for cratering are not the same numbers, so a single table cannot serve every purpose.
A kiloton is a mass unit everywhere else — why not here?
Because the phrase abbreviates “the energy released by one kiloton of TNT”, and that mass was never present. One kiloton is 109 g of equivalent, hence 109 × 4 184 = 4.184 × 1012 J, which the converter reports as 4 184 000 MJ, or 4.184 TJ. A device weighing a few tonnes can release fifteen kilotons — 6.276 × 1013 J — which is the clearest possible demonstration that the unit counts energy alone.
How does a megajoule figure turn into an overpressure at a given distance?
Through cube-root scaling. Blast charts are plotted against Z = R ÷ W1/3, where R is the standoff in metres and W the TNT-equivalent charge in kilograms, and any two events sharing a Z see the same peak overpressure. A 100 kg charge at 20 m gives Z = 20 ÷ 4.64 = 4.31; holding that same Z with a 1 000 kg charge pushes the standoff out to 43.1 m. Because the energy enters under a cube root, ten times the megajoules buys only 2.15 times the safe radius.
If real TNT releases 4.6 MJ per kilogram, is the convention wrong?
No — the two figures answer different questions. Measured detonation heat for trinitrotoluene lands nearer 4.6 MJ/kg, roughly 10 per cent above the defined 4.184 MJ/kg, and the exact value depends on confinement and on whether afterburning in air is counted. The convention deliberately ignores all of that so the yardstick cannot drift with the measurement technique. The one rule is not to mix them: state a yield on the defined basis or on a measured basis, and say which one was used.
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