diff --git a/views.py b/views.py
index 9d80b97..00f4bfa 100644
--- a/views.py
+++ b/views.py
@@ -10,6 +10,8 @@ import scipy.special as sp
 from astropy.coordinates import SkyCoord, Angle
 from astropy.stats import poisson_conf_interval
 
+from collections import defaultdict
+
 from django.db.models import Sum
 from django.shortcuts import get_object_or_404
 from rest_framework.views import APIView
@@ -225,6 +227,7 @@ class UpperLimitView(APIView):
         # COMPUTE COUNTS, BACKGROUND ESTIMATE, EXPOSURE
         # **************************************************************
 
+        # summing counts across all surveys
         N = sum(obj.counts for obj in source_pixels)
 
         Nnpix = len(source_pixels)
@@ -235,9 +238,21 @@ class UpperLimitView(APIView):
 
         B = Bcounts / Bnpix * Nnpix
 
-        tsum = sum(obj.exposure for obj in source_pixels)
+        # create a dict of exposures keyed by survey
+        t_by_survey = defaultdict(list)
 
-        t = tsum / Nnpix
+        for pixel in source_pixels:
+            t_by_survey[pixel.survey].append(pixel.exposure)
+
+        # create and populate a list of average exposures per survey
+        survey_averages = []
+
+        for survey_id, exposures in t_by_survey.items():
+            average_exposure = sum(exposures) / len(exposures)
+            survey_averages.append(average_exposure)
+
+        # sum them up across surveys
+        t = sum(survey_averages)
 
         # CONSTANTS
         # **************************************************************