diff --git a/builder/amazon/ebs/builder.go b/builder/amazon/ebs/builder.go
index b9277a5ab..50274ef87 100644
--- a/builder/amazon/ebs/builder.go
+++ b/builder/amazon/ebs/builder.go
@@ -72,6 +72,17 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 	session := session.New(config)
 	ec2conn := ec2.New(session)
 
+	// If the subnet is specified but not the AZ, try to determine the AZ automatically
+	if b.config.SubnetId != "" && b.config.AvailabilityZone == "" {
+		log.Printf("[INFO] Finding AZ for the given subnet '%s'", b.config.SubnetId)
+		resp, err := ec2conn.DescribeSubnets(&ec2.DescribeSubnetsInput{SubnetIds: []*string{&b.config.SubnetId}})
+		if err != nil {
+			return nil, err
+		}
+		b.config.AvailabilityZone = *resp.Subnets[0].AvailabilityZone
+		log.Printf("[INFO] AZ found: '%s'", b.config.AvailabilityZone)
+	}
+
 	// Setup the state bag and initial state for the steps
 	state := new(multistep.BasicStateBag)
 	state.Put("config", b.config)
diff --git a/builder/amazon/instance/builder.go b/builder/amazon/instance/builder.go
index 6b1726efb..96b8fa9cc 100644
--- a/builder/amazon/instance/builder.go
+++ b/builder/amazon/instance/builder.go
@@ -163,6 +163,17 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 	session := session.New(config)
 	ec2conn := ec2.New(session)
 
+	// If the subnet is specified but not the AZ, try to determine the AZ automatically
+	if b.config.SubnetId != "" && b.config.AvailabilityZone == "" {
+		log.Printf("[INFO] Finding AZ for the given subnet '%s'", b.config.SubnetId)
+		resp, err := ec2conn.DescribeSubnets(&ec2.DescribeSubnetsInput{SubnetIds: []*string{&b.config.SubnetId}})
+		if err != nil {
+			return nil, err
+		}
+		b.config.AvailabilityZone = *resp.Subnets[0].AvailabilityZone
+		log.Printf("[INFO] AZ found: '%s'", b.config.AvailabilityZone)
+	}
+
 	// Setup the state bag and initial state for the steps
 	state := new(multistep.BasicStateBag)
 	state.Put("config", &b.config)