diff --git a/homeassistant/util/unit_conversion.py b/homeassistant/util/unit_conversion.py
index 6d502ee6e6d8..9aa3084887ec 100644
--- a/homeassistant/util/unit_conversion.py
+++ b/homeassistant/util/unit_conversion.py
@@ -71,6 +71,10 @@ _DAYS_TO_SECS = 24 * _HRS_TO_SECS  # 1 day = 24 hours = 86400 seconds
 _POUND_TO_G = 453.59237
 _OUNCE_TO_G = _POUND_TO_G / 16
 
+# Pressure conversion constants
+_STANDARD_GRAVITY = 9.80665
+_MERCURY_DENSITY = 13.5951
+
 # Volume conversion constants
 _L_TO_CUBIC_METER = 0.001  # 1 L = 0.001 m³
 _ML_TO_CUBIC_METER = 0.001 * _L_TO_CUBIC_METER  # 1 mL = 0.001 L
@@ -211,9 +215,9 @@ class PressureConverter(BaseUnitConverter):
         PRESSURE_BAR: 1 / 100000,
         PRESSURE_CBAR: 1 / 1000,
         PRESSURE_MBAR: 1 / 100,
-        PRESSURE_INHG: 1 / 3386.389,
+        PRESSURE_INHG: 1 / (_IN_TO_M * 1000 * _STANDARD_GRAVITY * _MERCURY_DENSITY),
         PRESSURE_PSI: 1 / 6894.757,
-        PRESSURE_MMHG: 1 / 133.322,
+        PRESSURE_MMHG: 1 / (_MM_TO_M * 1000 * _STANDARD_GRAVITY * _MERCURY_DENSITY),
     }
     VALID_UNITS = {
         PRESSURE_PA,
diff --git a/tests/util/test_pressure.py b/tests/util/test_pressure.py
index 769c5aaf8016..f87b89df3f76 100644
--- a/tests/util/test_pressure.py
+++ b/tests/util/test_pressure.py
@@ -118,7 +118,7 @@ def test_convert_from_inhg():
         101.59167
     )
     assert pressure_util.convert(inhg, PRESSURE_INHG, PRESSURE_MMHG) == pytest.approx(
-        762.002
+        762
     )
 
 
@@ -126,23 +126,23 @@ def test_convert_from_mmhg():
     """Test conversion from mmHg to other units."""
     inhg = 30
     assert pressure_util.convert(inhg, PRESSURE_MMHG, PRESSURE_PSI) == pytest.approx(
-        0.580102
+        0.580103
     )
     assert pressure_util.convert(inhg, PRESSURE_MMHG, PRESSURE_KPA) == pytest.approx(
-        3.99966
+        3.99967
     )
     assert pressure_util.convert(inhg, PRESSURE_MMHG, PRESSURE_HPA) == pytest.approx(
-        39.9966
+        39.9967
     )
     assert pressure_util.convert(inhg, PRESSURE_MMHG, PRESSURE_PA) == pytest.approx(
-        3999.66
+        3999.67
     )
     assert pressure_util.convert(inhg, PRESSURE_MMHG, PRESSURE_MBAR) == pytest.approx(
-        39.9966
+        39.9967
     )
     assert pressure_util.convert(inhg, PRESSURE_MMHG, PRESSURE_CBAR) == pytest.approx(
-        3.99966
+        3.99967
     )
     assert pressure_util.convert(inhg, PRESSURE_MMHG, PRESSURE_INHG) == pytest.approx(
-        1.181099
+        1.181102
     )
diff --git a/tests/util/test_unit_conversion.py b/tests/util/test_unit_conversion.py
index ec839a6575c9..d74bacc66f8c 100644
--- a/tests/util/test_unit_conversion.py
+++ b/tests/util/test_unit_conversion.py
@@ -361,14 +361,14 @@ def test_power_convert(
         (30, PRESSURE_INHG, pytest.approx(101591.67), PRESSURE_PA),
         (30, PRESSURE_INHG, pytest.approx(1015.9167), PRESSURE_MBAR),
         (30, PRESSURE_INHG, pytest.approx(101.59167), PRESSURE_CBAR),
-        (30, PRESSURE_INHG, pytest.approx(762.002), PRESSURE_MMHG),
-        (30, PRESSURE_MMHG, pytest.approx(0.580102), PRESSURE_PSI),
-        (30, PRESSURE_MMHG, pytest.approx(3.99966), PRESSURE_KPA),
-        (30, PRESSURE_MMHG, pytest.approx(39.9966), PRESSURE_HPA),
-        (30, PRESSURE_MMHG, pytest.approx(3999.66), PRESSURE_PA),
-        (30, PRESSURE_MMHG, pytest.approx(39.9966), PRESSURE_MBAR),
-        (30, PRESSURE_MMHG, pytest.approx(3.99966), PRESSURE_CBAR),
-        (30, PRESSURE_MMHG, pytest.approx(1.181099), PRESSURE_INHG),
+        (30, PRESSURE_INHG, pytest.approx(762), PRESSURE_MMHG),
+        (30, PRESSURE_MMHG, pytest.approx(0.580103), PRESSURE_PSI),
+        (30, PRESSURE_MMHG, pytest.approx(3.99967), PRESSURE_KPA),
+        (30, PRESSURE_MMHG, pytest.approx(39.9967), PRESSURE_HPA),
+        (30, PRESSURE_MMHG, pytest.approx(3999.67), PRESSURE_PA),
+        (30, PRESSURE_MMHG, pytest.approx(39.9967), PRESSURE_MBAR),
+        (30, PRESSURE_MMHG, pytest.approx(3.99967), PRESSURE_CBAR),
+        (30, PRESSURE_MMHG, pytest.approx(1.181102), PRESSURE_INHG),
     ],
 )
 def test_pressure_convert(