You've just unboxed a new vacuum pump, or you're reading a lab report, and there it is: pressure listed in torr. But your reference chart, your doctor, or your equipment manual speaks in mmHg. Are these two units rivals? Do you need a calculator? Take a breath — this is one of the friendliest conversions in all of measurement science.

The Short Answer: They're Essentially the Same

Here's the headline before we go any deeper: one torr corresponds to one millimeter of mercury (mmHg). The two units were built to describe the same amount of pressure, so converting between them is about as close to a "no-op" as unit conversions get.

In day-to-day work — reading a gauge, logging vacuum data, checking a blood pressure reading — you can treat them as interchangeable. Just keep the number and swap the label.

What Exactly Is a Torr?

The torr is a unit of pressure named after Evangelista Torricelli, the Italian physicist who invented the mercury barometer in the 17th century. His famous experiment showed that the weight of the atmosphere could hold up a column of mercury roughly 760 millimeters tall at sea level.

From that discovery came a clean definition: a torr represents one 760th of a standard atmosphere. Because it traces its roots directly to that first barometer, the torr became the natural language of vacuum science — the field Torricelli accidentally founded.

What Exactly Is a Millimeter of Mercury (mmHg)?

Millimeter of mercury, usually written mmHg, describes the pressure exerted by a column of mercury exactly one millimeter tall. Picture a thin glass tube filled with liquid metal: the push that column delivers at its base is one mmHg.

This definition made mmHg wonderfully intuitive for early scientists. Need more pressure? The mercury rises. Less pressure? It falls. You could literally see the measurement.

Why Do Both Units Exist If They Match?

Fair question. The answer comes down to precision and tradition.

• The torr is fixed by definition. It's tied mathematically to the standard atmosphere, so it never wobbles.
• mmHg is tied to a physical substance. Mercury's density shifts slightly with temperature, and gravity varies slightly across the Earth. So a "millimeter of mercury" can depend on where and when you measure it.

In practice, that distinction is so small that virtually no instrument you'll ever touch can tell the difference. Scientists kept both names because each community had already adopted its favorite: vacuum engineers say torr, clinicians say mmHg.

Converting Torr to mmHg: The Easiest Math You'll Do Today

No multiplication. No memorized constants. The conversion works like this:

1. Take your value in torr.
2. Keep the number exactly as it is.
3. Relabel it as mmHg.

Going the other direction? Identical process. A reading of 75 torr equals 75 mmHg, and 640 mmHg equals 640 torr.

A Quick Worked Example

Suppose a freeze-drying protocol calls for a chamber pressure of 100 torr, but your gauge displays mmHg. Your target on that gauge is simply 100 mmHg. Set it, run it, done.

Torr to mmHg Conversion Table

Here are common values side by side so you can verify at a glance:

Where You'll Meet These Units in Real Life

Vacuum Science and Industry

The torr dominates high-vacuum work. Semiconductor manufacturing, sputter coating, electron microscopes, and freeze-drying all specify their operating pressures in torr because the scale fits the range those processes demand.

Medicine

mmHg owns the clinic. Blood pressure readings like 120 over 80 are measured in mmHg, as is intraocular pressure during eye exams and blood gas analysis in critical care. When your doctor inflates that cuff, they're working in Torricelli's units — they just call them something else.

Meteorology and Everyday Weather

Older barometers and some aviation contexts still reference millimeters of mercury when describing atmospheric pressure, a direct nod to the instrument that started it all.

Common Pitfalls to Avoid

• Don't confuse mmHg with other "column" units. Centimeters of water (cmH₂O) and inches of mercury (inHg) are different units entirely, even though they sound similar.
• Don't mix up torr with pascals or millibars. Those belong to other pressure scales and require genuine calculations to convert.
• Watch your abbreviations. "mmHg" always means millimeters of mercury. If you see "mm H₂O," that's a water column — a much smaller pressure.

The Bottom Line

Torr and mmHg are two names for practically the same quantity of pressure, born from the same barometer experiment centuries apart in usage. Convert between them by keeping the number and changing the label — nothing more. Whether you're calibrating a vacuum furnace or decoding a blood pressure chart, you now speak both dialects fluently.

Need to move between these units and other pressure scales, like pascals or psi? Use the converter above to handle any pairing instantly and accurately.