Converting Between Metric and Non-Metric Units:

 A Quick-Reference Guide for Chemists

Article By Global Instruments

Introduction: Why Chemists Still Need Both Systems

Chemistry, as a discipline, lives almost entirely within the metric system — more precisely, within the

International System of Units (SI). Reagent bottles are labeled in grams and milliliters, balances read out in milligrams, and journal articles report concentrations in moles per liter. Yet despite this, working chemists routinely run into non-metric units: a legacy piece of equipment calibrated in inches, a decades-old procedure written in pounds and gallons, an industrial process specification in psi, or a collaborator's data reported in degrees Fahrenheit. The pharmaceutical and petrochemical industries in the United States, in particular, still operate with a mix of metric lab work and imperial process engineering.

For a chemist, unit conversion is not an abstract math exercise — it is a daily, practical necessity, and one where errors carry real consequences. A misplaced decimal in a molarity calculation can ruin a synthesis; a mishandled pressure conversion in a high-pressure reaction can be dangerous. This guide is built as a working reference: the core metric-to-imperial conversion factors a chemist actually needs, organized by the quantities that come up most often in the lab, along with the reasoning and worked examples needed to use them correctly and safely.

The Foundation: SI Base Units Chemists Rely On

Before converting to non-metric units, it helps to be precise about the metric baseline. The SI system chemists use daily rests on these base and derived units:

• Mass — kilogram (kg), though chemists work mostly in grams (g) and milligrams (mg)
• Length — meter (m), with centimeters (cm) and millimeters (mm) common at the bench
• Volume — liter (L), a non-SI unit but universally accepted alongside milliliters (mL) and microliters (µL)
• Temperature — kelvin (K) is the SI unit, though Celsius (°C) is used for everyday lab reporting
• Amount of substance — mole (mol), the unit unique to chemistry
• Pressure — pascal (Pa), though bar and atmosphere remain common in practice
• Energy — joule (J), with kilojoules (kJ) and calories (cal) both still seen in thermodynamics

Every conversion in this guide treats these as the "home base" units and builds outward toward the imperial and US customary units chemists most often encounter.

Mass Conversions

Mass is the most frequently converted quantity in a chemistry lab, particularly when working from older procedures, industrial-scale batch sheets, or shipping and safety documentation that still uses pounds.

From	To	Multiply By
grams (g)	ounces (oz)	0.035274
kilograms (kg)	pounds (lb)	2.20462
pounds (lb)	kilograms (kg)	0.453592
ounces (oz)	grams (g)	28.3495
milligrams (mg)	grains (gr)	0.0154324

Worked example: A safety data sheet lists a reagent's maximum recommended batch size as 50 lb. To prepare an equivalent scaled-down lab batch, you need this in kilograms.

50 lb × 0.453592 kg/lb = 22.68 kg

Practical note: Analytical balances in almost every modern lab are calibrated in metric units, so mass conversions to imperial units are usually needed only when interfacing with legacy equipment, older literature, or non-metric shipping and regulatory paperwork rather than for routine bench work.

Volume Conversions

Volume conversions are where chemists most often encounter US customary units, particularly gallons and fluid ounces, since much of the chemical manufacturing and shipping industry in the United States still specifies tank and drum capacities in gallons.

From	To	Multiply By
liters (L)	US gallons (gal)	0.264172
US gallons (gal)	liters (L)	3.78541
milliliters (mL)	US fluid ounces (fl oz)	0.033814
US fluid ounces (fl oz)	milliliters (mL)	29.5735
liters (L)	US quarts (qt)	1.05669

Worked example: A pilot-plant reactor is rated for a maximum working volume of 55 US gallons. What is that in liters, for use in a metric-based process control system?

55 gal × 3.78541 L/gal = 208.20 L

Important caveat: Always confirm whether a source is using the US gallon or the imperial (UK) gallon — they differ by roughly 20%, since one imperial gallon equals about 1.20095 US gallons. Chemical suppliers publishing internationally sometimes specify volumes ambiguously as simply "gallons," which is precisely the kind of ambiguity dimensional analysis cannot resolve on its own; it must be clarified with the original source before converting.

Length Conversions

Length conversions come up less often in routine benchwork but matter for glassware specifications, tubing and fitting sizes (particularly in chromatography and gas-handling systems, where fittings are frequently specified in inches), and equipment dimensions.

From	To	Multiply By
centimeters (cm)	inches (in)	0.393701
inches (in)	centimeters (cm)	2.54
meters (m)	feet (ft)	3.28084
feet (ft)	meters (m)	0.3048
millimeters (mm)	inches (in)	0.0393701

Worked example: A chromatography column fitting is specified as 1/4 inch outer diameter. To confirm compatibility with metric tubing, convert to millimeters.

0.25 in × 25.4 mm/in = 6.35 mm

This is a case where an exact conversion factor matters more than most: the inch is defined as exactly 2.54 centimeters (25.4 millimeters), by international agreement since 1959, so this conversion carries no rounding uncertainty at all — unlike most of the other factors in this guide, which are conversions between systems built on independently defined units and are therefore only precise to the number of significant figures shown.

Temperature Conversions

Temperature is unusual among the quantities in this guide because the conversions are not simple multiplications — they involve both a scaling factor and an offset, since the zero points of the Celsius, Fahrenheit, and Kelvin scales do not align.

Celsius to Fahrenheit: °F = (°C × 9/5) + 32

Fahrenheit to Celsius: °C = (°F − 32) × 5/9

Celsius to Kelvin: K = °C + 273.15

Worked example: A distillation procedure written for a US audience specifies a column head temperature of 176°F. What is that in Celsius, for use with a metric-calibrated thermocouple?

(176 − 32) × 5/9 = 144 × 5/9 = 80°C

Common pitfall: Because temperature conversion involves addition, not just multiplication, it cannot be handled by the pure multiply-by-a-fraction dimensional analysis method used for the other quantities in this guide. Treat temperature conversions as a separate category, and always double-check which direction the offset goes — a forgotten "+32" or a sign error is one of the most common mistakes chemists make when adapting older Fahrenheit-based procedures.

Pressure Conversions

Pressure conversions matter enormously for safety, particularly around gas cylinders, autoclaves, and vacuum systems, where US-manufactured equipment is frequently rated in psi (pounds per square inch) even in labs that otherwise work entirely in metric units.

From	To	Multiply By
pascals (Pa)	psi	0.000145038
psi	pascals (Pa)	6894.76
bar	psi	14.5038
psi	bar	0.0689476
atmospheres (atm)	psi	14.6959

Worked example: A gas regulator's maximum delivery pressure is rated at 3000 psi. Converting to bar for a metric-labeled system:

3000 psi × 0.0689476 bar/psi = 206.84 bar

Safety note: Pressure conversion errors are among the most dangerous mistakes a chemist can make, since a misconverted maximum pressure rating can lead to equipment failure. When working with pressurized gas systems, cross-check any converted value against the equipment's original manufacturer documentation rather than relying solely on a calculated conversion.

Energy Conversions

Thermodynamics and calorimetry are where energy units cross between systems most often, particularly because the calorie remains common in nutritional and some older thermochemical contexts even though the joule is the SI unit.

From	To	Multiply By
joules (J)	calories (cal)	0.239006
calories (cal)	joules (J)	4.184
kilojoules (kJ)	BTU	0.947817
BTU	kilojoules (kJ)	1.05506

Worked example: A bomb calorimetry result reports a heat of combustion of 2500 cal for a sample. In joules, for use in a standard thermochemistry calculation:

2500 cal × 4.184 J/cal = 10,460 J = 10.46 kJ

Concentration and Density: Composite Units

Chemists frequently need to convert between concentration expressions that mix metric mass or volume units with non-metric equivalents, especially when translating between US industrial specifications and laboratory-standard molarity.

Example — converting a density-based specification: A specification sheet lists a solvent's density as 8.34 lb/gal. Convert to the standard chemistry unit of g/mL.

8.34 lb/gal × 453.592 g/lb × 1 gal/3.78541 L × 1 L/1000 mL = 0.999 g/mL

This example is worth studying closely because it shows the real power of the dimensional analysis approach discussed in the companion article on unit conversion: a composite unit like pounds per gallon converts cleanly into grams per milliliter simply by chaining together conversion factors and canceling units step by step, without needing to memorize a single direct pound-per-gallon-to-gram-per-milliliter factor.

A Practical Workflow for the Chemistry Bench

1. Identify the unit system mismatch first. Before doing any calculation, confirm which units your source data uses and which units your target system, instrument, or calculation requires.
2. Separate temperature from everything else. Remember that temperature conversions require an offset, not just a multiplication factor.
3. Watch for US vs. imperial ambiguity in gallons, ounces, and tons — these differ between American and British definitions and are a frequent source of silent errors.
4. Use exact conversion factors where they exist (such as the inch-to-centimeter relationship) and note the precision limits of approximate factors elsewhere.
5. Cross-check safety-critical conversions, especially pressure and temperature limits for pressurized or heated equipment, against original manufacturer specifications rather than relying on a single calculated value.
6. Carry appropriate significant figures through the conversion and round only at the final step, consistent with the precision of the original measurement.

Conclusion

Even in a discipline as thoroughly metric as chemistry, the non-metric world is never entirely out of reach — legacy equipment, cross-border collaboration, industrial-scale process specifications, and decades of published literature all ensure that pounds, gallons, psi, and Fahrenheit continue to surface in a working chemist's day. Rather than treating these as occasional annoyances to be looked up individually, it is far more reliable to keep a small, trusted set of conversion factors on hand, apply them through a consistent method, and pay special attention to the handful of conversions — temperature offsets, US versus imperial volume units, and safety-critical pressure ratings — where a moment of carelessness can lead to a real error. With a clear reference table and a disciplined approach to unit tracking, moving between metric and non-metric systems becomes a routine, low-risk part of laboratory work rather than a recurring source of uncertainty.

Dimensional Analysis 101: