diff --git a/README.md b/README.md
index f905259..76be7d5 100644
--- a/README.md
+++ b/README.md
@@ -3,6 +3,8 @@
 <img src="img/gcp-ip-addresses.png" width="32%"></img>
 <img src="img/azure-ip-addresses.png" width="32%"></img>
 
+`aws_region_ip_sizes.py` fetches AWS `ip-ranges.json`, collapses overlapping IPv4 CIDRs, and prints unique IP counts by region.
+
 # [compute / memory unit prices by virtual machine type](https://instances.vantage.sh/)
 <img src="img/ec2-unit-prices.png" width="100%"></img>
 <img src="img/gcp-unit-prices.png" width="49%"></img>
diff --git a/aws_region_ip_sizes.py b/aws_region_ip_sizes.py
new file mode 100644
index 0000000..b05d4a7
--- /dev/null
+++ b/aws_region_ip_sizes.py
@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+from collections import defaultdict
+from ipaddress import collapse_addresses, ip_network
+from json import load
+from urllib.request import urlopen
+
+
+AWS_IP_RANGES_URL = "https://ip-ranges.amazonaws.com/ip-ranges.json"
+
+
+def load_aws_prefixes(url=AWS_IP_RANGES_URL):
+    with urlopen(url) as response:
+        return load(response)["prefixes"]
+
+
+def unique_ipv4_counts_by_region(prefixes):
+    regions = defaultdict(list)
+    for prefix in prefixes:
+        if "ip_prefix" not in prefix:
+            continue
+        regions[prefix["region"]].append(ip_network(prefix["ip_prefix"]))
+
+    return {
+        region: sum(network.num_addresses for network in collapse_addresses(networks))
+        for region, networks in regions.items()
+    }
+
+
+def main():
+    counts = unique_ipv4_counts_by_region(load_aws_prefixes())
+    total = sum(counts.values())
+
+    for region, count in sorted(counts.items(), key=lambda item: item[1], reverse=True):
+        print(f"{region}: {count / total:.2%} ({count:,})")
+    print(f"total: {total // 1_000_000} million")
+
+
+if __name__ == "__main__":
+    main()