PRESIDENT OBAMA’S main pitch for the pending nuclear deal with Iran is that it would extend the “breakout time” necessary for Iran to produce enough enriched uranium for a nuclear weapon. In a recent interview with NPR, he said that the current breakout time is “about two to three months by our intelligence estimates.” By contrast, he claimed, the pending deal would shrink Iran’s nuclear program, so that if Iran later “decided to break the deal, kick out all the inspectors, break the seals and go for a bomb, we’d have over a year to respond.”

Unfortunately, that claim is false, as can be demonstrated with basic science and math. By my calculations, Iran’s actual breakout time under the deal would be approximately three months — not over a year. Thus, the deal would be unlikely to improve the world’s ability to react to a sudden effort by Iran to build a bomb.

Breakout time is determined by three primary factors: the number and type of centrifuges; the enrichment of the starting material; and the amount of enriched uranium required for a nuclear weapon. Mr. Obama seems to make rosy assumptions about all three.

Most important, in the event of an overt attempt by Iran to build a bomb, Mr. Obama’s argument assumes that Iran would employ only the 5,060 centrifuges that the deal would allow for uranium enrichment, not the roughly 14,000 additional centrifuges that Iran would be permitted to keep mainly for spare parts. Such an assumption is laughable. In a real-world breakout, Iran would race, not crawl, to the bomb.