NMR converter
Convert magnet field strength to resonance frequency. Pick the nucleus first: carbon resonates at a quarter of hydrogen’s frequency in the same magnet.
That's about the same as a 400 MHz spectrometer. Hydrogen resonates at about 400 MHz in that field.
What it means
What does it resonate at?
Frequencies are for the bare nucleus. Chemical shifts move them by parts per million, which is the entire information content of a spectrum.
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Three things worth knowing
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Protons: 42.577 MHz per tesla
So a 400 MHz instrument is 9.4 T, a 600 is 14.1 T, and a clinical 3 T scanner works at 128 MHz.
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Why stronger magnets are better
Signal grows faster than noise with field strength, and peaks spread further apart, so a higher field buys both sensitivity and resolution. It also costs a great deal more.
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MHz is not the field
Saying "a 400 MHz magnet" is shorthand for the proton frequency. The field is 9.4 T, and every other nucleus in it has a different frequency.
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Background
About NMR frequency
A spectrometer described as "400 MHz" is a 9.4 tesla magnet: the number quoted is the frequency at which hydrogen nuclei resonate inside it, at 42.577 MHz per tesla.
Every nucleus has its own ratio. In that same magnet, carbon-13 resonates at 100 MHz and nitrogen-15 at 41.