Warhammer 40K Exterminatus: Calculating the Energy to Destroy a Planet

Exterminatus being performed on Dolumar IV using Cyclonic Torpedos in Warhammer 40,000: Fire Warrior

Exterminatus is the ultimate sanction of war and punishment, used by the Imperium only when all other options have failed. Declaring Exterminatus on a world authorizes its total and irreversible destruction.

This action can be carried out using several types of weapons, each with different effects and levels of devastation. In this post, we aim to estimate the yield and characteristics of these weapons based on evidence from novels, animated series, and video game cinematics.

For consistency, we will assume that planets subjected to Exterminatus have Earth-like properties—especially size and mass—unless explicit information states otherwise.

Orbital Bombardment

As the name suggests, this method involves a sustained, large-scale bombardment by an entire fleet against the target planet. It is the most commonly used form of Exterminatus.

Orbital bombardments employ “conventional” weaponry such as lance batteries, kinetic strikes, magma bombs, and similar arms. While individually less extreme than Cyclonic Torpedoes, their cumulative effect is sufficient to render a planet uninhabitable or completely devastated.

Several canonical examples of this method are mentioned throughout the lore.

• "Every weapon in the battleship’s arsenal was prepared and oriented down at the surface; torpedo arrays filled with warshots that could atomise whole continents in a single strike, energy cannons capable of boiling off oceans, kinetic killers that could behead mountains through the brute force of their impact. This was only the power of the ship itself; then there was the minor fleet of auxiliary craft aboard it, wings of fighters and bombers that could come screaming down into Dagonet’s atmosphere on plumes of white fire. Swift death bringers that could raze cities, burn nations." NEMESIS, pag. 561.

• "The majestic form of the Vae Victus slowly angled its massive bulk towards the surface of Pavonis, powerful energies building in her forward linear accelerators. Few men knew the awesome power of destruction the captain of a starship possessed: the power to level cities and crack continents. For all that the captains of the Imperial Navy might strut and boast of the capabilities of their ponderous warships, there was nothing that could compete with the sheer destructive speed and efficiency of a Space Marine strike cruiser". NIGHTBRINGER pag. 220.

• "A mighty Repulsive-class Grand Cruiser with powerful reactors and heavy armour in sloping facets of adamantine and ceramite scores of metres thick, the vessel carried a weight of armament and ordnance that could reduce a continent to ruins with a single salvo." BLACK CRUSADE

• "At a great distance, the vessel would have resembled the head of a mace, filigreed with gold and black iron. At close range, a city's worth of spires and gantries reached out, many of them glowing with the light of thousands of windows, others concealing nests of weapons capable of killing a continent". FLIGHT OF THE EISENSTEIN

• "The attacking fleet is escorting Extermiantors, ships capable of laying waste to entire planetary populations or even obliterating all life on a world in a matter of hours". BATTLEFLEET GOTHIC

• "Meanwhile the massed guns of the fleet continued to pound the planet, until many monasteries were reduced to miles-wide craters, angry magma spewing from the raw wounds in the planet's crust. The very surface of Caliban began to crack and heave under the strain of the bombardment, the fury of the Dark Angels blinding them to the devastation they were wreaking on their own world." CODEX DARK ANGELS, 4TH EDITION

• "Lord Admiral Tiberius had wanted to level the entire palace with orbital bombardment, but Uriel had resisted such a plan, knowing that the vast forces the Vae Victus could unleash would level everything within fifty kilometres of the palace. The greatly reduced yield on the magma bombs had struck with precisely the correct force, and though there was certain to be some collateral casualties, Uriel hoped that that they had been kept to a minimum".NIGHTBRINGER

• "By turning their massive lances and batteries of macrocannons on a planet and unleashing a storm of deadly ordnance, a ship can perform a Planetary Bombardment. Unspeakably devastating, a planetary bombardment from a single ship can lay waste to armies and cities, while a concentrated bombardment from a battle group can turn an entire continent to ash".BATTLEFLEET KORONUS

• "Armaments that could hurl energy hundreds of thousands of kilometres across space now turned their power on the planet's surface in an awesome display of destructive capability, gouging wounds hundreds of meters deep into the rock and soil of the hills in search of the silos, command bunkers and generator caverns buried there."EXECUTION HOUR

There are many more references to planetary bombardments carried out by the Imperial Navy and the Adeptus Astartes. However, only the clearest and easiest to quantify examples are included here to avoid overloading the post with citations.

We can clearly identify two types of conventional munitions with explicitly described effects: magma bombs and lance strikes, both of which allow reasonable energy estimates.

Magma Bombs

According to the cited material, a single magma bomb can level everything within a 50 km radius around a fortified palace. Using the Glasstone & Dolan / Kingery–Bulmash airblast scaling law:

Air-blast radius (≈20 psi overpressure):

$$Y = \left(\frac{x}{0.28}\right)^3$$

Where:

• Y is the yield in kilotons of TNT

• x is the blast radius in kilometers

• 0.28 corresponds to a 20-psi overpressure (enough to destroy reinforced structures)

Substituting the values:

$$Y = (50 / 0.28)^3 \approx 5{,}694{,}241 \text{ kilotons}$$

That is ≈5,694 megatons of TNT. Since the target was a heavily armored palace, the real yield may be even higher.

• Four Goth-class cruisers (Intolerable, Invincible, and Righteous Force) waited four parsecs from the target. Each ship carried 100 Hellfire nuclear missiles. Each missile had 122 warheads, each with a firepower of 5 GT. If the boarding action fails, the nuclear missiles will turn the ship into dust.” Space Hulk rulebook (scenario book)

This aligns well with the stated output of Hellfire missiles, each carrying 122 warheads, with ~5 gigatons per warhead. That results in 610 gigatons (610,000 megatons) per missile—more than enough for continent-level devastation and large-scale ocean vaporization during a sustained bombardment.

Lance weapons

Lance strikes are described as penetrating several hundred meters into the surface. Taking a conservative depth of 300 meters and an impact area equivalent to a city block (~20,000 m²), the resulting crater can be approximated as a cylinder.

Crater volume:

$$V = 6{,}000{,}000 \text{ m}^3$$

Assuming average rock density (~2,700 kg/m³) and ~10 MJ/kg to vaporize rock:

$$E = (6{,}000{,}000 \times 2{,}700) \times 10{,}000{,}000$$$$E \approx 1.62 \times 10^{17} \text{ joules}$$

That equals ≈38 megatons of TNT per lance shot.

While lance weapons are primarily designed for deep penetration rather than wide-area destruction, their energy is still sufficient to obliterate and vitrify entire city sectors.

This represents only the conventional arsenal available to most Imperial, Astartes, and Inquisition vessels. Next, we move on to the weapons reserved for a select few ships—those capable of destroying a planet with a single strike.

Cyclonic Torpedoes

Typhon Primaris during Exterminatus in Dawn of War 2

Each Cyclonic Torpedo is stated to possess enough power to devastate and melt a planet’s rocky crust, as shown during Exterminatus on Typhon Primaris, Dolumar IV, and Cyrene. In addition to their raw yield, the extreme speed at which the explosion propagates across the planet is a key factor. To estimate their power, we must consider two elements: the energy required to melt a planetary crust and the expansion speed of the blast.

Earth’s crust has an average thickness of ~40 km and a mass of about 2.7 × 10²² kg, roughly 0.5% of Earth’s total mass.

Energy required to melt the crust

• Mass: 2.7 × 10²² kg

• Average specific heat: ~1,100 J/kg·°C

• Melting temperature: ~1,200 °C

• Initial temperature: ~20 °C

• Temperature change: 1,180 °C

• Latent heat of fusion: ~400,000 J/kg

Energy to raise the temperature:

Q₁ = 2.7 × 10²² × 1,100 × 1,180 ≈ 3.5 × 10²⁸ J

Energy for phase change (solid → liquid):

Q₂ = 2.7 × 10²² × 400,000 ≈ 1.0 × 10²⁸ J

Total energy:

Q = Q₁ + Q₂ ≈ 4.5 × 10²⁸ joules,

equivalent to ~10 exatons of TNT (10 trillion megatons).

Explosion expansion speed

To engulf an Earth-sized planet, the blast must travel roughly half the circumference:

• Earth’s circumference: 40,075 km

• Distance covered: ~20,037 km

On Typhon Primaris, the explosion spreads across the planet in ~10 seconds, implying a velocity of:

~2,000 km/s (2,000,000 m/s)

This speed is sufficient to generate a shockwave capable of tearing mountains from the crust and ejecting them into space as debris.

Comparison

The Chicxulub impact, responsible for the mass extinction 65 million years ago, produced shockwave velocities of up to ~6 km/s. By comparison, a Cyclonic Torpedo–based Exterminatus is orders of magnitude more destructive, both in energy output and shockwave propagation.

To further support this estimate, we can examine additional examples from novels and codices.

• "Another method involves the use of cyclonic torpedoes, a category of weapons that use varying means of scouring the world's surface with nucleonic fire, raw plasma, or deadly radiation." TACTICA IMPERIALIS

• "Shortly afterwards, a flotilla of Naval and Inquisition vessels had joined the Ravenous Spirit [Strike Cruiser] in orbit and had proceeded to launch an unrelenting barrage of lance strikes, mass drivers and cyclone torpedoes, reducing the once green world to a primeval, molten state. "As I recall, you were the one who requested the assistance of the Inquisition in the performance of an exterminatus on cyrene - the systematic annihilation of all life on the planet - genocide by another name". DAWN OF WAR

• "A bombardment had begun, and the people of Dagonet's capital feared it was the end of the world. They knew so little of the reality of things, however. High above in orbit, it was only the warship Thanato that fired on the city, and even then it was not with the vessel's most powerful cannons. The people did not know that a fleet of craft were poised in silence around their sister ship, watchful and waiting. Had all the vessels of the Warmaster's flotilla unleashed their killpower, then indeed those fears would have come true; the planet's crust cracked, the continents sliced open. Perhaps those things would happen, soon enough — but for now it was sufficient for the Thanato to hurl inert kinetic kill-rods down through the atmosphere, the sky-splitting shriek of their passage climaxed by a lowing thunder as the warshots obliterated power stations, military compounds and the vast mansion-houses of the noble clans. From the ground it seemed like wanton destruction; from orbit, it was a shrewd and surgical pattern of attack. … Dagonet was all but dead now, her surface a mosaic of burning cities, churned oceans and glassed wastelands. And yet this was a show of restraint from the Sons of Horus; had they wished it, the world could have suffered the fate of many that had defied the Warmaster, cracked open by cyclonic torpedo barrages shot into key tectonic target sites, remade into a sphere of molten earth. Instead Dagonet was being prepared. It would be of use to the Warmaster and his march to victory." NEMESIS

• "Less than twenty-four hours later more than a hundred and ninety million people were dead – over ninety-eight per cent of the doomed world's population. The cruisers and battleships assigned to the Forty-seventh Expeditionary Fleet anchored at high orbit, and for twelve hours unleashed their payload upon the condemned, storm-wracked planet. Cyclonic torpedoes and concentrated hellfire broadsides pierced the storm clouds spanning the planet. Entire continents had disappeared in flames." TALES OF HERESY

• "The methods of delivering this ultimate sanction vary depending on the source of the order and the organisation that delivers the sentence, but all are equally uncompromising. The Adeptus Astartes tend towards delivering an Exterminatus device to the planet's surface. such a mission takes the carefully picked kill team right to the heart of the foe, where they can ensure the absolute death of the world. By contrast, the Imperial Navy prefer sustained bombardments, using battleships to hammer the world until its very structure collapses. The grim corvettes of the Inquisition favour esoteric weapons such as cyclonic torpedoes and atmospheric incinerators."  SIXTH EDITION RULEBOOK

• "All had heard of Exterminatus with fusion torpedoes, virus bombs and mass drivers, but to know the enemy had the ability to destroy an entire planet, not just all life on it, must have been the most chilling thought that any naval crewman had ever faced." BATTLEFLEET GOTHIC

The aforementioned quotes are sufficient to confirm the idea of the total destruction of a planet's surface through cyclonic torpedoes, turning it into a mass of molten rock that briefly glows like a star.

Grey Knights performing Exterminatus on a Nurgle-corrupted world   in Warhammer 40,000: Chaos Gate – Daemonhunters

Nova cannon.

A Nova Cannon deployed on the prow of a Dominator-class Cruiser of the Imperial Navy.

• “A nova cannon is a huge weapon, normally mounted in the prow of a ship… It fires a projectile at incredible velocity, using graviometric impellers to accelerate it to close to light speed. The projectile implodes at a preset distance, unleashing a force more potent than a dozen plasma bombs.” Battlefleet Gothic rulebook , page 22.

• "These projectiles vary more than the nature of the cannons themselves, ranging from sophisticated plasma warheads which burn with the ferocity of a small star for a fraction of a second, to implosive devices which exert destructive gravitational forces upon all those caught within several thousand kilometres of the detonation." -Rogue Trader RPG : Battlefleet Koronos, page 15.

A cannon capable of firing projectiles at 90–99% the speed of light is among the most terrifying weapons in the Imperial Navy’s arsenal. It is stated to be powerful enough to obliterate battleships and heavy cruisers in a single hit, despite their void shields. This is especially impressive considering cruisers measure 8–10 km in length, while battleships can exceed 12 km, depending on class.

 Nova Cannon destroying a chaos warship

But how powerful is a single shot?

A useful reference is that 1 kg of mass accelerated to light speed carries ~3.65 × 10¹⁶ joules (≈8.7 megatons of TNT). The projectile fired by this weapon has an estimated diameter of 50 meters, giving it a volume of ~65,449 m³. Assuming a steel-like density (7,850 kg/m³), the projectile’s mass is approximately 5.14 × 10⁸ kg.

A direct kinetic impact at relativistic speed would therefore release:

~1.8 × 10²⁵ joules,
equivalent to ~4.3 billion megatons of TNT.

conclusion.

Even using conservative assumptions, Exterminatus weapons operate at extreme energy scales. From cyclonic torpedoes to relativistic cannons, the Imperium possesses the means to erase worlds, not just conquer them.