PSIG to PSIA Conversion Calculator — Gauge to Absolute Pressure
PSIA = PSIG + atmospheric pressure. Enter your gauge reading and the local atmospheric pressure (or altitude for an ISA-based estimate) to get the absolute pressure in psia, kPa, bar and atm.
psig
ft
psi
PSIA = PSIG + atmospheric pressure
- 1
Atmospheric pressure (manual entry)
14.696 - 2
Absolute pressure (PSIA = PSIG + P_atm)
30 + 14.696 = 44.6960PSIA is always measured from a perfect vacuum — add atmospheric pressure to the gauge reading.
How does this calculator work?
PSIA = PSIG + atmospheric pressure (14.696 psi at sea level). Gauge pressure is what most instruments read; absolute pressure is needed for gas-law calculations. Atmospheric pressure drops ≈ 0.5 psi per 1 000 ft altitude. Enter PSIG and your elevation to get PSIA, kPa, bar and atm.
Formula
How this is calculated
Most pressure gauges measure relative to the surrounding atmosphere rather than from an absolute vacuum. This "gauge pressure" (psig) is what tyre gauges, boiler gauges, and air compressors report. To get the true (absolute) pressure that appears in equations of state and thermodynamic tables — such as the ideal gas law PV = nRT — you add the ambient atmospheric pressure: PSIA = PSIG + P_atm.
At sea level the standard atmospheric pressure is 101 325 Pa = 14.696 psi (ISO 2533). This value decreases with altitude according to the International Standard Atmosphere (ISA) troposphere model: P_atm = 14.696 × (1 − 6.876 × 10⁻⁶ × h_ft)^5.256, valid from sea level to approximately 36 089 ft (11 km). At 5 000 ft, P_atm ≈ 12.23 psi; at 10 000 ft, ≈ 10.11 psi. Ignoring the altitude correction introduces errors of ≈ 3 % per 5 000 ft.
Vacuum-gauge readings are negative psig (below atmospheric). For example, −10 psig at sea level = 14.696 − 10 = 4.696 psia. A perfect vacuum is 0 psia (= −14.696 psig). Vacuum is also expressed as "inches of mercury vacuum" (inHg vac); 0 inHg vac = 0 psig = atmospheric, and 29.92 inHg vac ≈ −14.696 psig = 0 psia.
Frequently asked questions
Gas laws (PV = nRT, Boyle's law, etc.) require absolute pressure. Using gauge pressure in those equations gives wrong results. For flow calculations, tank storage, and thermodynamics, always convert to PSIA first. For relative comparisons (tyre inflation, HVAC differential) gauge pressure is more convenient.
The ISA troposphere model gives P_atm = 14.696 × (1 − 6.876 × 10⁻⁶ × altitude_ft)^5.256 psi. Example: at 5 000 ft ≈ 12.23 psia; at 10 000 ft ≈ 10.11 psia; at Denver, CO (5 280 ft) ≈ 12.15 psia. Enter your altitude in the field to get an automatic estimate.
No. PSIA is an absolute scale measured from a perfect vacuum, so it cannot go below 0 psia. A reading of 0 psia represents a complete vacuum. If your calculation gives a negative PSIA it means the entered gauge pressure (negative vacuum pressure) exceeds the atmospheric pressure, which is physically impossible in practice.
Also known as
TG we-Calculate Editorial Team. (2026). PSIG to PSIA Conversion Calculator — Gauge to Absolute Pressure [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/psig-to-psia-conversion-calculator
TG we-Calculate Editorial Team. "PSIG to PSIA Conversion Calculator — Gauge to Absolute Pressure." TG we-Calculate. 2026. https://we-calculate.com/calculator/psig-to-psia-conversion-calculator.
TG we-Calculate Editorial Team, "PSIG to PSIA Conversion Calculator — Gauge to Absolute Pressure," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/psig-to-psia-conversion-calculator
@misc{wecalculate_psig_to_psia_conversion_calculator, title = {PSIG to PSIA Conversion Calculator — Gauge to Absolute Pressure}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/psig-to-psia-conversion-calculator}}, year = {2026}, note = {TG we-Calculate} }
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