diff --git a/wagtail/contrib/postgres_search/apps.py b/wagtail/contrib/postgres_search/apps.py
index 8d0b93fbce..530fdb72a1 100644
--- a/wagtail/contrib/postgres_search/apps.py
+++ b/wagtail/contrib/postgres_search/apps.py
@@ -1,11 +1,10 @@
-from __future__ import absolute_import, unicode_literals
+from __future__ import absolute_import, division, unicode_literals
 
 from django.apps import AppConfig
 from django.core.checks import Error, Tags, register
 
 from .utils import (
-    BOOSTS_WEIGHTS, WEIGHTS_COUNT, WEIGHTS_VALUES, determine_boosts_weights,
-    get_postgresql_connections)
+    BOOSTS_WEIGHTS, WEIGHTS_VALUES, determine_boosts_weights, get_postgresql_connections)
 
 
 class PostgresSearchConfig(AppConfig):
@@ -21,9 +20,6 @@ class PostgresSearchConfig(AppConfig):
                           id='wagtail.contrib.postgres_search.E001')]
 
         BOOSTS_WEIGHTS.extend(determine_boosts_weights())
-        sorted_boosts_weights = sorted(BOOSTS_WEIGHTS, key=lambda t: t[0])
-        max_weight = sorted_boosts_weights[-1][0]
+        max_weight = BOOSTS_WEIGHTS[0][0]
         WEIGHTS_VALUES.extend([v / max_weight
-                               for v, w in sorted_boosts_weights])
-        for _ in range(WEIGHTS_COUNT - len(WEIGHTS_VALUES)):
-            WEIGHTS_VALUES.insert(0, 0)
+                               for v, w in reversed(BOOSTS_WEIGHTS)])
diff --git a/wagtail/contrib/postgres_search/tests/test_backend.py b/wagtail/contrib/postgres_search/tests/test_backend.py
index 9ce885f0c5..df1a745ede 100644
--- a/wagtail/contrib/postgres_search/tests/test_backend.py
+++ b/wagtail/contrib/postgres_search/tests/test_backend.py
@@ -7,6 +7,8 @@ from django.utils.six import StringIO
 from wagtail.tests.search.models import SearchTest
 from wagtail.wagtailsearch.tests.test_backends import BackendTests
 
+from ..utils import BOOSTS_WEIGHTS, WEIGHTS_VALUES, determine_boosts_weights, get_weight
+
 
 class TestPostgresSearchBackend(BackendTests, TestCase):
     backend_path = 'wagtail.contrib.postgres_search.backend'
@@ -40,3 +42,31 @@ class TestPostgresSearchBackend(BackendTests, TestCase):
         results = self.backend.search('world', SearchTest)
         self.assertSetEqual(set(results), {self.testa,
                                            self.testd.searchtest_ptr})
+
+    def test_weights(self):
+        self.assertListEqual(BOOSTS_WEIGHTS,
+                             [(10, 'A'), (2, 'B'), (0, 'C'), (0, 'D')])
+        self.assertListEqual(WEIGHTS_VALUES, [0, 0, 0.2, 1.0])
+
+        self.assertEqual(get_weight(15), 'A')
+        self.assertEqual(get_weight(10), 'A')
+        self.assertEqual(get_weight(9.9), 'B')
+        self.assertEqual(get_weight(2), 'B')
+        self.assertEqual(get_weight(1.9), 'C')
+        self.assertEqual(get_weight(0), 'C')
+        self.assertEqual(get_weight(-1), 'D')
+
+        self.assertListEqual(determine_boosts_weights([1]),
+                             [(1, 'A'), (0, 'B'), (0, 'C'), (0, 'D')])
+        self.assertListEqual(determine_boosts_weights([-1]),
+                             [(-1, 'A'), (-1, 'B'), (-1, 'C'), (-1, 'D')])
+        self.assertListEqual(determine_boosts_weights([-1, 1, 2]),
+                             [(2, 'A'), (1, 'B'), (-1, 'C'), (-1, 'D')])
+        self.assertListEqual(determine_boosts_weights([0, 1, 2, 3]),
+                             [(3, 'A'), (2, 'B'), (1, 'C'), (0, 'D')])
+        self.assertListEqual(determine_boosts_weights([0, 0.25, 0.75, 1, 1.5]),
+                             [(1.5, 'A'), (1, 'B'), (0.5, 'C'), (0, 'D')])
+        self.assertListEqual(determine_boosts_weights([0, 1, 2, 3, 4, 5, 6]),
+                             [(6, 'A'), (4, 'B'), (2, 'C'), (0, 'D')])
+        self.assertListEqual(determine_boosts_weights([-2, -1, 0, 1, 2, 3, 4]),
+                             [(4, 'A'), (2, 'B'), (0, 'C'), (-2, 'D')])
diff --git a/wagtail/contrib/postgres_search/utils.py b/wagtail/contrib/postgres_search/utils.py
index 0d8393c0d4..68b500e135 100644
--- a/wagtail/contrib/postgres_search/utils.py
+++ b/wagtail/contrib/postgres_search/utils.py
@@ -1,4 +1,4 @@
-from __future__ import absolute_import, unicode_literals
+from __future__ import absolute_import, division, unicode_literals
 
 import operator
 import re
@@ -8,6 +8,7 @@ from django.apps import apps
 from django.db import connections
 from django.db.models import Q
 from django.utils.lru_cache import lru_cache
+from django.utils.six.moves import zip_longest
 
 from wagtail.wagtailsearch.index import Indexed, RelatedFields, SearchField
 
@@ -94,25 +95,34 @@ BOOSTS_WEIGHTS = []
 WEIGHTS_VALUES = []
 
 
-def determine_boosts_weights():
+def get_boosts():
     boosts = set()
     for model in apps.get_models():
         if issubclass(model, Indexed):
             for search_field in get_search_fields(model.get_search_fields()):
                 boost = search_field.boost
-                boosts.add(0 if boost is None else boost)
+                if boost is not None:
+                    boosts.add(boost)
+    return boosts
+
+
+def determine_boosts_weights(boosts=()):
+    if not boosts:
+        boosts = get_boosts()
+    boosts = list(sorted(boosts, reverse=True))
+    min_boost = boosts[-1]
     if len(boosts) <= WEIGHTS_COUNT:
-        return zip(reversed(sorted(boosts)), WEIGHTS)
-    min_boost = min(boosts)
-    max_boost = max(boosts)
-    boost_step = (max_boost - min_boost) / WEIGHTS_COUNT
-    return [(min_boost + (i * boost_step), weight)
-            for i, weight in zip(range(WEIGHTS_COUNT), WEIGHTS)]
+        return list(zip_longest(boosts, WEIGHTS, fillvalue=min(min_boost, 0)))
+    max_boost = boosts[0]
+    boost_step = (max_boost - min_boost) / (WEIGHTS_COUNT - 1)
+    return [(max_boost - (i * boost_step), weight)
+            for i, weight in enumerate(WEIGHTS)]
 
 
 def get_weight(boost):
     if boost is None:
-        boost = 0
+        return WEIGHTS[-1]
     for max_boost, weight in BOOSTS_WEIGHTS:
         if boost >= max_boost:
             return weight
+    return weight