diff --git a/python/lsst/ip/diffim/subtractImages.py b/python/lsst/ip/diffim/subtractImages.py
index 073bb78c..2cebb25c 100644
--- a/python/lsst/ip/diffim/subtractImages.py
+++ b/python/lsst/ip/diffim/subtractImages.py
@@ -20,6 +20,7 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 from astropy import units as u
+from astropy.stats import gaussian_fwhm_to_sigma
 import numpy as np
 
 import lsst.afw.detection as afwDetection
@@ -612,10 +613,9 @@ def runKernelSourceDetection(self, template, science):
         """
         kernelSize = self.makeKernel.makeKernelBasisList(
             self.templatePsfSize, self.sciencePsfSize)[0].getWidth()
-        sigmaToFwhm = 2*np.log(2*np.sqrt(2))
         candidateList = self.makeKernel.makeCandidateList(template, science, kernelSize,
                                                           candidateList=None,
-                                                          sigma=self.sciencePsfSize/sigmaToFwhm)
+                                                          sigma=gaussian_fwhm_to_sigma*self.sciencePsfSize)
         sources = self.makeKernel.selectKernelSources(template, science,
                                                       candidateList=candidateList,
                                                       preconvolved=False,
diff --git a/python/lsst/ip/diffim/utils.py b/python/lsst/ip/diffim/utils.py
index dc8d28fc..c7bc12b3 100644
--- a/python/lsst/ip/diffim/utils.py
+++ b/python/lsst/ip/diffim/utils.py
@@ -27,13 +27,16 @@
            ]
 
 import itertools
-import numpy as np
 import os
 import requests
-import lsst.geom as geom
+
+from astropy.stats import gaussian_sigma_to_fwhm
+import numpy as np
+
 import lsst.afw.detection as afwDetection
 import lsst.afw.image as afwImage
 import lsst.afw.math as afwMath
+import lsst.geom as geom
 from lsst.pex.exceptions import InvalidParameterError, RangeError
 import lsst.pipe.base
 from lsst.utils.logging import getLogger
@@ -129,12 +132,12 @@ def getPsfFwhm(psf, average=True, position=None):
     if position is None:
         position = psf.getAveragePosition()
     shape = psf.computeShape(position)
-    sigmaToFwhm = 2*np.log(2*np.sqrt(2))
 
     if average:
-        return sigmaToFwhm*shape.getTraceRadius()
+        return gaussian_sigma_to_fwhm*shape.getTraceRadius()
     else:
-        return [sigmaToFwhm*np.sqrt(shape.getIxx()), sigmaToFwhm*np.sqrt(shape.getIyy())]
+        return [gaussian_sigma_to_fwhm*np.sqrt(shape.getIxx()),
+                gaussian_sigma_to_fwhm*np.sqrt(shape.getIyy())]
 
 
 def evaluateMeanPsfFwhm(exposure: afwImage.Exposure,
diff --git a/tests/test_subtractTask.py b/tests/test_subtractTask.py
index 9052f719..dbf31271 100644
--- a/tests/test_subtractTask.py
+++ b/tests/test_subtractTask.py
@@ -980,7 +980,7 @@ def test_metadata_metrics(self):
         self.assertTrue(np.isnan(maglim_template_offimage))
         # Test that given the provided fallbackPsfSize, the diffim limiting
         # magnitude is calculated correctly.
-        maglim_template_offimage = 28.3173123103493
+        maglim_template_offimage = 28.182284789714952
         fluxlim_template_offimage = (maglim_template_offimage*u.ABmag).to_value(u.nJy)
         maglim_offimage = (np.sqrt(fluxlim_science**2 + fluxlim_template_offimage**2)*u.nJy).to(u.ABmag).value
         self.assertEqual(subtractTask_offimage.metadata['diffimLimitingMagnitude'], maglim_offimage)
diff --git a/tests/test_utilsCalculations.py b/tests/test_utilsCalculations.py
index b9be3a8d..d41320ea 100644
--- a/tests/test_utilsCalculations.py
+++ b/tests/test_utilsCalculations.py
@@ -19,6 +19,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
+from astropy.stats import gaussian_sigma_to_fwhm
 import numpy as np
 
 import lsst.geom
@@ -45,13 +46,12 @@ def test_angleMean(self):
     def test_getPsfFwhm(self):
         """Calculation of FWHM from a realization of the PSF
         """
-        sigmaToFwhm = 2*np.log(2*np.sqrt(2))
 
         def make_and_check_psf(xKsize, yKsize, sigma):
             psf = measAlg.SingleGaussianPsf(xKsize, yKsize, sigma)
             psfSize = getPsfFwhm(psf)
             psfSize2d = getPsfFwhm(psf, average=False)
-            self.assertFloatsAlmostEqual(sigma*sigmaToFwhm, psfSize, rtol=0.01)
+            self.assertFloatsAlmostEqual(gaussian_sigma_to_fwhm*sigma, psfSize, rtol=0.01)
             self.assertFloatsAlmostEqual(psfSize, psfSize2d[0], rtol=0.01)
             self.assertFloatsAlmostEqual(psfSize, psfSize2d[1], rtol=0.01)