diff --git a/optipdf b/optipdf
index fc32636..3a63f3e 100755
--- a/optipdf
+++ b/optipdf
@@ -105,6 +105,22 @@ if [[ ${#FILES[@]} -eq 0 ]]; then
     exit 2
 fi
 
+check_required_binaries() {
+    local BIN MISSING=()
+    for BIN; do
+        if ! type -t "$BIN" &>/dev/null; then
+            MISSING+=("$BIN")
+        fi
+    done
+    if [[ ${#MISSING[@]} -gt 0 ]]; then
+        printf 'Error: You are missing required programs:\n' >&2
+        for BIN in "${MISSING[@]}"; do
+            printf -- '- %s\n' "$BIN" >&2
+        done
+        exit 2
+    fi
+}
+
 file_size() {
     stat \
         --format='%s' \
@@ -116,43 +132,10 @@ file_size() {
 
 # produces a human-readable size from the byte count passed to it
 hr_size() (
-    declare -i BYTES=$1
-
-    #UNITS=(B KB MB GB TB PB EB ZB YB) # shell math can only go so far...
-    UNITS=(B KB MB GB TB)
-    FACT=1024
-    THRESH=9/10
-    DECIMALS=1
-    DECIMALS_FACTOR=$(( 10 ** DECIMALS ))
-
-    # cycle through units from largest to smallest, exiting when it finds the
-    # largest applicable unit
-    for (( EXP = ${#UNITS[@]} - 1; EXP > -1; --EXP )); do
-        # check if the unit is close enough to the unit's size, within the
-        # threshold
-        if [[ $BYTES -gt $((FACT ** EXP * $THRESH)) ]]; then
-            # we found the applicable unit
-
-            # must multiply by a factor of 10 here to not truncate
-            # the given number of decimal places after the point
-            HR_VAL=$(( BYTES * DECIMALS_FACTOR / FACT ** EXP ))
-
-            # put the decimal point in
-            if [[ $DECIMALS -gt 0 ]]; then
-                HR_VAL=$(( HR_VAL / DECIMALS_FACTOR )).$(( HR_VAL % DECIMALS_FACTOR ))
-            fi
-
-            HR_UNIT=${UNITS[$EXP]}
-            break
-        fi
-    done
-
-    if [[ -z $HR_UNIT ]]; then
-        HR_VAL=$BYTES
-        HR_UNIT=${UNITS[0]}
-    fi
-
-    printf '%g %s\n' "$HR_VAL" "$HR_UNIT"
+    numfmt \
+        --to=iec \
+        --suffix=B \
+        "$@"
 )
 
 setup_tempdir() {
@@ -233,6 +216,9 @@ use_smaller() {
 
 }
 
+check_required_binaries \
+    qpdf
+
 QPDF_ARGS=(
     --compression-level=9
     --deterministic-id