I assume we're talking about man made explosions as opposed to natural. If the latter, a volcano is the likely suspect.

Having asked this question myself, the answer I have most often found is no one is sure. Most of the leading candidates were accidents, usually military storage explosions, that didnt have much in the way of testing going on nearby for metric verification.

My guess is the Dupont Powder Mill explosion of 1907. Reports were that the concussion could be felt almost 200 miles away! Buildings were damaged at least a mile away.

During the US civil war the battle of the crater was fought in a hole made by 300 or so kegs of gunpowder. According to Wikipedia the Powder Mill explosion was closer to 40,000 kegs of gunpowder. That gives you a pretty decent idea of the kind of scale we're talking about.

While that surely isn't definite I have to think it was close. There were many large explosions that could be possible candidates. While a ship explosion is possible I have trouble believing any ship ever carried more than 40,000 kegs of gunpowder's worth of explosive power up until the advent of nuclear weapons.

Assuming you mean man-made explosions, according to Time: Disasters that Shook the World, the biggest pre-nuclear explosion was the Halifax Explosion. A munitions ship bound for Europe collided with a freighter in the Canadian port city and destroyed about 400 acres of the city

It's difficult to say, given the imprecision of measurements taken before the invention of accurate instruments and scales designed to calculate large explosion.

The biggest pre-July 20, 1945 explosion was, of course, the Big Bang, but I'm guessing you're referring to man-made explosions.

There are two principal candidates: The 1907 Dupont Powder Mill explosion (explained by another commenter) and the 1917 Halifax explosion. The Dupont Powder Mill explosion involved a large amount of gunpowder. Gunpowder, when it comes to explosive energy, is less powerful than the equivalent mass of TNT. Thus, we have instances where a smaller amount of TNT or other explosive could create a blast greater than the Dupont Powder Mill explosion.

The 1917 Halifax explosion took place when a Belgian relief ship, the Imo, collided with a French munitions ship, the Mont Blanc, in Halifax Harbour and set it ablaze. The Mont Blanc was almost literally a floating bomb. It was loaded with TNT, guncotton, picric accid, benzol and monochlorobenzol. In effect, it was carrying 2,925 tons of high explosives. On Dec. 6, 1917, all of it exploded.

Let me share an excerpt from Laura Mac Donald's Curse of the Narrows:

"Altogether 2,925 tons or 5.85 million pounds of powder exploded, giving off over 9,000 (degrees Fahrenheit) of heat. All 6,880,627 pounds of the Mont Blanc's iron hull shot up over a thousand feet, roiling within the initial flame ball until much of it vaporized. The anchor snapped its chain and separated, sending the 1,140-pound shank whirling through the skies over Richmond Hill and across the city for 2.35 miles ... The aft canon shot off the stern and arced over the harbor ... for another three miles, landing next to a small lake."

In an instant, most of Halifax was destroyed. More than 2,000 people were killed. Six thousand were left homeless, and 10,000 were injured. Many of the injured had severe eye and facial lacerations, caused because they had gathered at their windows to watch the Mont Blanc burn in the harbor, not knowing the cargo it carried.

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