diff --git a/src/libmain/shared.hh b/src/libmain/shared.hh
index 8e4861232..ab36b7c05 100644
--- a/src/libmain/shared.hh
+++ b/src/libmain/shared.hh
@@ -52,23 +52,7 @@ template<class N> N getIntArg(const string & opt,
 {
     ++i;
     if (i == end) throw UsageError(format("'%1%' requires an argument") % opt);
-    string s = *i;
-    N multiplier = 1;
-    if (allowUnit && !s.empty()) {
-        char u = std::toupper(*s.rbegin());
-        if (std::isalpha(u)) {
-            if (u == 'K') multiplier = 1ULL << 10;
-            else if (u == 'M') multiplier = 1ULL << 20;
-            else if (u == 'G') multiplier = 1ULL << 30;
-            else if (u == 'T') multiplier = 1ULL << 40;
-            else throw UsageError(format("invalid unit specifier '%1%'") % u);
-            s.resize(s.size() - 1);
-        }
-    }
-    N n;
-    if (!string2Int(s, n))
-        throw UsageError(format("'%1%' requires an integer argument") % opt);
-    return n * multiplier;
+    return parseSize<N>(*i, allowUnit);
 }
 
 
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 9f239bff3..fd09f8f2b 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -359,7 +359,7 @@ bool statusOk(int status);
 
 
 /* Parse a string into an integer. */
-template<class N> bool string2Int(const string & s, N & n)
+template<typename N> bool string2Int(const string & s, N & n)
 {
     if (string(s, 0, 1) == "-" && !std::numeric_limits<N>::is_signed)
         return false;
@@ -368,6 +368,27 @@ template<class N> bool string2Int(const string & s, N & n)
     return str && str.get() == EOF;
 }
 
+template<typename N>
+N parseSize(std::string s, bool allowUnit = true)
+{
+    N multiplier = 1;
+    if (allowUnit && !s.empty()) {
+        char u = std::toupper(*s.rbegin());
+        if (std::isalpha(u)) {
+            if (u == 'K') multiplier = 1ULL << 10;
+            else if (u == 'M') multiplier = 1ULL << 20;
+            else if (u == 'G') multiplier = 1ULL << 30;
+            else if (u == 'T') multiplier = 1ULL << 40;
+            else throw Error("invalid unit specifier '%1%'", u);
+            s.resize(s.size() - 1);
+        }
+    }
+    N n;
+    if (!string2Int(s, n))
+        throw Error("'%s' is not an integer", s);
+    return n * multiplier;
+}
+
 /* Parse a string into a float. */
 template<class N> bool string2Float(const string & s, N & n)
 {