Herodotus gives about 20,000 Persian deaths at Thermopylae.
The figure belongs to Herodotus’s account of the three-day battle in 480 BC, when Leonidas and a Greek coalition blocked Xerxes’ invasion force at the narrow pass. After the Persian army found a mountain path around the Greek position, the defenders were encircled and destroyed. Herodotus says the Persian losses reached roughly 20,000, while Greek losses were about 4,000 by his accounting.
This is an ancient literary estimate, not a modern casualty register. Herodotus’s totals for Persian forces are often considered exaggerated by modern historians, who generally place Xerxes’ army far below the millions claimed by ancient sources. The 20,000 figure should therefore be understood as Herodotus’s reported estimate, often phrased as “up to” 20,000, rather than a securely verified count.
A common mix-up is confusing the Persian death toll with the famous 300 Spartans. The Spartans were only part of the Greek force, which initially numbered several thousand, including 700 Thespians and 400 Thebans.