Unit converter for engineers

How the values are converted

Every unit is defined by an exact factor to its SI unit, taken from the definition of the unit. A typed decimal times such a factor is an exact fraction, so a conversion is exact; the result is rounded once, for display, to the significant digits you choose, and the table says whether the shown value is exact or rounded. No floating point is involved.

Defining values

• Length and mass: international yard and pound (1959): 1 in = 25.4 mm, 1 ft = 0.3048 m, 1 lb = 0.45359237 kg exactly. 1 µin = 25.4 nm, 1 mil = 25.4 µm. US gallon = 231 in³ = 3.785411784 L; imperial gallon = 4.54609 L.
• Force: standard gravity g_n = 9.80665 m/s² (exact by definition), so 1 kgf = 9.80665 N and 1 lbf = 0.45359237 kg × g_n = 4.4482216152605 N exactly. The local gravity is not used.
• Pressure: 1 bar = 100 000 Pa; 1 atm = 101 325 Pa; 1 Torr = 101 325 / 760 Pa = 133.3223684… Pa; 1 psi = 1 lbf / in² = 6894.757293168… Pa (the exact fraction 8896443230521 / 1290320000 Pa); 1 kgf/cm² (technical atmosphere) = 98 066.5 Pa; 1 N/mm² = 1 MPa.
• Liquid columns are the conventional units: mmHg with mercury at 13 595.1 kg/m³ and g_n, 1 mmHg = 133.322387415 Pa (1 inHg = 3386.388640341 Pa); mmH₂O and inH₂O with water at 1000 kg/m³, 1 mmH₂O = 9.80665 Pa and 1 inH₂O = 249.08891 Pa. Tables that use water at 4 °C (999.97 kg/m³) or at 60 °F differ by up to about 0.1 %; this convention is the one of the conventional (“technical”) units.
• Torque: force × lever arm with the same exact factors: 1 lbf·ft = 1.3558179483314004 N·m, 1 lbf·in = 0.1129848290276167 N·m, 1 kgf·m = 9.80665 N·m, 1 ozf·in = 1/16 lbf·in.
• Energy and power: International Table calorie 4.1868 J and BTU 1055.05585262 J; thermochemical calorie 4.184 J (food); 1 hp (mechanical) = 550 ft·lbf/s = 745.69987158227022 W; 1 PS = 75 kgf·m/s = 735.49875 W.
• Temperature: K = °C + 273.15, K = (°F + 459.67) × 5/9, K = °R × 5/9. A temperature below 0 K is refused. Temperature differences are a separate quantity without the offsets: 1 °C = 1 K, 1 °F = 1 °R = 5/9 K.

Gauge, absolute and differential pressure

• Absolute pressure = gauge pressure + atmospheric pressure. You say which one you typed; with an atmospheric pressure (default 101.325 kPa, the standard atmosphere) every row also shows the other one, and the headline can be switched to it.
• An absolute pressure cannot be negative; a gauge pressure below a full vacuum is refused. A typed difference (Δp across a filter, a column of water) has no reference and is never shifted.
• The units themselves do not carry “g” or “a”: write bar(g) / bar(a), psig / psia on drawings and in reports.

Flow and viscosity

• Volumetric flow (m³/h, L/min, US gpm, imperial gpm, cfm …) and mass flow (kg/h, lb/h …) are linked only through a density you give: mass flow = volumetric flow × density. Kinematic and dynamic viscosity likewise: dynamic = kinematic × density (1 cP = 1 mPa·s; 1 cSt = 1 mm²/s).
• Gas flow at reference conditions (the Gas flow part): at the same mass flow an ideal gas obeys Q_ref = Q_actual × (p_actual / p_ref) × (T_ref / T_actual) with absolute pressures and temperatures. Dry gas, no compressibility factor Z (below about 10 bar and near room temperature, Z of air is within about 1 % of 1), no humidity correction. The reference conditions are always chosen by you; nothing is assumed.
• Reference conditions offered (unverified): Reference conditions for gas volumes as commonly quoted (DIN 1343, ISO 13443, ISO 1217, IUPAC, CAGI, US gas practice); recalled from general practice, the standards themselves were not read. Check which conditions your flow meter, compressor data sheet or supplier uses.

Surface roughness

• Ra, Rq, Rz … in µm, µin, nm or mm: 1 µin = 0.0254 µm exactly (so Ra 1.6 µm = 62.9921 µin; drawings round it to 63 µin).
• The N grade numbers (unverified): Roughness grade numbers N1–N12 with their Ra values as given in older editions of ISO 1302 (before 2002) and widely reproduced in drawing handbooks; recalled from general practice, the standard was not read. ISO 1302:2002 no longer uses grade numbers (recalled). The grade shown is the smallest one whose Ra value the given Ra does not exceed.
• No conversion between Ra and Rz (or Rt, Rmax): they describe different features of the same profile, and their ratio depends on the process (often quoted between about 4 and 10). A converted value could pass or fail a part wrongly, so the tool does not offer one.

Hardness (approximate)

• Hardness scales measure different things (indentation depth or area under different loads and indenters); there is no exact conversion. Conversion tables are empirical and valid for a material group only.
• Table used (unverified): Approximate hardness conversion numbers for non-austenitic steels, written from memory of the tables of ASTM E140 (Tables 1 and 2) as widely reproduced; the standard (and ISO 18265) was not read. Every value must be compared with the current standard before use. Scope: Non-austenitic steels: carbon and alloy steels in the as-forged, annealed, normalized, or quenched and tempered condition, of uniform hardness. Not for austenitic (stainless) steels, cast iron, non-ferrous metals, case-hardened, cold-worked or surface-treated parts.
• Linear interpolation between two neighbouring rows of one table; a conversion never chains two tables; a value outside the table, or where the table has no value for the other scale (HB above about HRC 58), is refused. Every result carries a warning, in the CSV too.
• No tensile strength is estimated: the table has no strength column, and a strength from hardness is no substitute for a tensile test.

Display

• Rounding to the chosen significant digits, half away from zero. Notation: auto (plain from 0.0001 to below 10⁹, scientific outside), plain, scientific (1.23e-4) or engineering (exponent a multiple of 3: 123e-6).
• The factor column “1 x =” shows the exact decimal when it has at most the shown digits, otherwise the rounded factor followed by “…”; the definition column says how the factor is defined. Temperatures show a formula instead.
• Decimal sign: as typed (a comma in the value gives commas in the results) or chosen; the CSV then uses “;” between columns.

Worked examples (the tool’s examples)

1. 6 bar gauge = 600 000 Pa / 6894.757293168… Pa = 87.0226 psi (gauge). With the standard atmosphere the absolute pressure is 701.325 kPa = 7.01325 bar = 101.719 psi.
2. 100 m³/h of compressed air at 7 bar(g), 20 °C, atmosphere 101.325 kPa, to normal conditions (0 °C, 101.325 kPa): 100 × (801.325 / 101.325) × (273.15 / 293.15) = 736.891 Nm³/h.
3. Ra 1.6 µm = 1.6 / 0.0254 = 62.9921 µin, grade N7 in the table.
4. HRC 50 ≈ 513 HV ≈ 481 HB in the (unverified) steel table: approximate only.
5. The batch example turns tightening torques in lbf·in into N·m: 9 lbf·in = 1.01686 N·m, 60 lbf·in = 6.77909 N·m, 420 lbf·in = 47.4536 N·m.

Common mistakes

• Converting a gauge pressure to an absolute unit without adding the atmosphere (or a vacuum reading in mbar(a) as if it were gauge).
• Converting a temperature difference with the offset: a rise of 10 °C is 18 °F, not 50 °F.
• Quoting Nm³/h, SCFM or “standard litres” without the reference conditions: the same number can differ by 7 % or more between conventions.
• Using kgf or lbf as mass units, or local gravity for kgf: the technical units use g_n = 9.80665 m/s² by definition.
• Taking a converted hardness or an Ra ↔ Rz ratio as a measurement for acceptance.

Input and files

• Values with a decimal point or comma, an optional exponent (2.5e-3) and sign; thousands separators are refused (1,000 could mean one or one thousand).
• Batch: one value per line, a label before it allowed; CSV, TSV or TXT files (UTF-8, UTF-16 or Windows-1257) up to 20 MB are read in this browser only. Excel workbooks are not opened here: copy the column and paste it. Up to 500 lines are shown on the page; the CSV holds all.
• Results CSV (UTF-8, “;” between columns with a decimal comma) for every part and one settings JSON for all; nothing is stored by this file.
• Checked against an independent script: exact fractions in Python for every unit pair (1,552 pairs), cross-checked with the pint library, and the table interpolation re-implemented there.

Questions

How many psi is 6 bar?

6 bar = 87.0226 psi. 1 psi is one pound-force on one square inch: 0.45359237 kg × 9.80665 m/s² / (0.0254 m)² = 6894.757293168… Pa, so 1 bar (100 000 Pa) = 14.5038 psi. If the 6 bar is a gauge reading, the absolute pressure at the standard atmosphere (101.325 kPa) is 7.01325 bar = 101.719 psi.

What is the difference between gauge and absolute pressure?

Absolute pressure is measured from a perfect vacuum, gauge pressure from the surrounding atmosphere: absolute = gauge + atmospheric. A tyre or a compressed-air line is usually read as gauge (bar(g), psig), a vacuum or a gas law calculation needs absolute (bar(a), psia). The converter asks which one you typed and shows the other one with the atmospheric pressure you give (101.325 kPa by default).

Why is a temperature difference converted differently from a temperature?

A temperature reading sits on a scale with an offset: 20 °C is 68 °F (× 9/5 + 32). A difference has no offset: a rise of 20 °C is a rise of 36 °F (× 9/5 only), and a tolerance of ±5 °C is ±9 °F. The converter has two separate quantities for this, so a span is never shifted by 32 °F.

What does Nm³/h mean, and how do I convert it to actual m³/h?

Nm³/h is a gas flow expressed at “normal” reference conditions, most often 0 °C and 101.325 kPa (DIN 1343); “standard” conditions are often 15 °C or 20 °C, or 60 °F and 14.696 psia for SCF. The same mass of gas fills a different volume at the operating pressure and temperature: Q_ref = Q_actual × (p_actual / p_ref) × (T_ref / T_actual) for an ideal gas, with absolute values. 100 m³/h of air at 7 bar(g) and 20 °C is about 737 Nm³/h. Always state which reference conditions a figure uses.

Can I convert Ra to Rz, or HRC to tensile strength?

Not reliably, so the converter does not. Ra and Rz describe different features of a profile and their ratio depends on the process. Hardness scales are converted only approximately, for steel, from an empirical table (marked unverified here, with a warning on every result); a value outside the table is refused. Measure the parameter or the scale the drawing specifies, and use a tensile test for strength.