diff --git a/CHANGES.md b/CHANGES.md
index 697c07e..909a866 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -7,6 +7,7 @@
 * `walker` and `codewalker` can now be used with `NONE` to remove elements
 * Improve `walker` performance by 70% by replacing clojure.walk implementation with custom recursive path
 * Extend `ALL` to work on records (navigate to key/value pairs)
+* Add ability to declare a function for end index of `srange-dynamic` that takes in the result of the start index fn. Use `end-fn` macro to declare this function (takes in 2 args of [collection, start-index]). Functions defined with normal mechanisms (e.g. `fn`) will still only take in the collection as an argument.
 * Workaround ClojureScript bug that emits warnings for vars named the same as a private var in cljs.core (in this case `NONE`, added as private var to cljs.core with 1.9.562)
 * For ALL transforms on maps, interpret transformed key/value pair of size < 2 as removal
 * Bug fix: Fix incorrect inline compilation when a dynamic function invocation is nested in a data structure within a parameter to a navigator builder
diff --git a/src/clj/com/rpl/specter.cljc b/src/clj/com/rpl/specter.cljc
index 2383b86..3eee0a6 100644
--- a/src/clj/com/rpl/specter.cljc
+++ b/src/clj/com/rpl/specter.cljc
@@ -452,7 +452,12 @@
              embed (vec (for [[t p] extensions] [t `(quote ~p)]))]
          `(extend-protocolpath*
            ~(protpath-sym protpath)
-           ~embed)))))
+           ~embed)))
+
+    (defmacro end-fn [& args]
+      `(n/->SrangeEndFunction (fn ~@args)))
+
+    ))
 
 
 
@@ -710,9 +715,11 @@
   srange-dynamic
   [start-fn end-fn]
   (select* [this structure next-fn]
-    (n/srange-select structure (start-fn structure) (end-fn structure) next-fn))
+    (let [s (start-fn structure)]
+      (n/srange-select structure s (n/invoke-end-fn end-fn structure s) next-fn)))
   (transform* [this structure next-fn]
-    (n/srange-transform structure (start-fn structure) (end-fn structure) next-fn)))
+    (let [s (start-fn structure)]
+      (n/srange-transform structure s (n/invoke-end-fn end-fn structure s) next-fn))))
 
 
 (defnav
diff --git a/src/clj/com/rpl/specter/navs.cljc b/src/clj/com/rpl/specter/navs.cljc
index 3bb7a6e..5cd904f 100644
--- a/src/clj/com/rpl/specter/navs.cljc
+++ b/src/clj/com/rpl/specter/navs.cljc
@@ -659,3 +659,12 @@
              []
              [v])
            ))))))
+
+(defrecord SrangeEndFunction [end-fn])
+
+;; done this way to maintain backwards compatibility
+(defn invoke-end-fn [end-fn structure start]
+  (if (instance? SrangeEndFunction end-fn)
+    ((:end-fn end-fn) structure start)
+    (end-fn structure)
+    ))
diff --git a/test/com/rpl/specter/core_test.cljc b/test/com/rpl/specter/core_test.cljc
index cea2603..bab6ca4 100644
--- a/test/com/rpl/specter/core_test.cljc
+++ b/test/com/rpl/specter/core_test.cljc
@@ -10,7 +10,7 @@
                       select-first transform setval replace-in
                       select-any selected-any? collected? traverse
                       multi-transform path dynamicnav recursive-path
-                      defdynamicnav traverse-all satisfies-protpath?]]))
+                      defdynamicnav traverse-all satisfies-protpath? end-fn]]))
   (:use
     #?(:clj [clojure.test :only [deftest is]])
     #?(:clj [clojure.test.check.clojure-test :only [defspec]])
@@ -21,7 +21,7 @@
                     select-first transform setval replace-in
                     select-any selected-any? collected? traverse
                     multi-transform path dynamicnav recursive-path
-                    defdynamicnav traverse-all satisfies-protpath?]]))
+                    defdynamicnav traverse-all satisfies-protpath? end-fn]]))
 
 
 
@@ -1561,6 +1561,22 @@
     (is (= (assoc f :b 1 :c 2) (transform [(s/walker (complement record?)) s/FIRST] (fn [k] (if (= :a k) :b :c)) f)))
     ))
 
+(def MIDDLE
+  (s/comp-paths
+    (s/srange-dynamic
+      (fn [aseq] (long (/ (count aseq) 2)))
+      (end-fn [aseq s] (if (empty? aseq) 0 (inc s))))
+    s/FIRST
+    ))
+
+(deftest srange-dynamic-test
+  (is (= 2 (select-any MIDDLE [1 2 3])))
+  (is (identical? s/NONE (select-any MIDDLE [])))
+  (is (= 1 (select-any MIDDLE [1])))
+  (is (= 2 (select-any MIDDLE [1 2])))
+  (is (= [1 3 3] (transform MIDDLE inc [1 2 3])))
+  )
+
 #?(:clj
   (do
     (defprotocolpath FooPP)