diff --git a/lexer.go b/lexer.go
index c91f863..93fb0d3 100644
--- a/lexer.go
+++ b/lexer.go
@@ -571,6 +571,7 @@ func DurationValue(s string, step int64) (int64, error) {
 	}
 	isMinus := false
 	d := float64(0)
+	s = strings.ToLower(s)
 	for len(s) > 0 {
 		n := scanSingleDuration(s, true)
 		if n <= 0 {
@@ -656,6 +657,7 @@ func scanSingleDuration(s string, canBeNegative bool) int {
 	if len(s) == 0 {
 		return -1
 	}
+	s = strings.ToLower(s)
 	i := 0
 	if s[0] == '-' && canBeNegative {
 		i++
diff --git a/lexer_test.go b/lexer_test.go
index 47cea27..1ab9f83 100644
--- a/lexer_test.go
+++ b/lexer_test.go
@@ -513,6 +513,14 @@ func TestPositiveDurationSuccess(t *testing.T) {
 	f("1.23", 45, 1230)
 	f("0.56", 12, 560)
 	f(".523e2", 21, 52300)
+
+	// duration suffixes in mixed case
+	f("1Ms", 45, 1)
+	f("1mS", 45, 1)
+	f("1M", 45, 1*60*1000)
+	f("1H", 45, 1*60*60*1000)
+	f("1D", 45, 1*24*60*60*1000)
+	f("1Y", 45, 1*365*24*60*60*1000)
 }
 
 func TestPositiveDurationError(t *testing.T) {
@@ -588,6 +596,13 @@ func TestDurationSuccess(t *testing.T) {
 	f("1.23", 45, 1230)
 	f("-0.56", 12, -560)
 	f("-.523e2", 21, -52300)
+
+	f("1Ms", 10, 1)
+	f("1mS", 10, 1)
+	f("1M", 10, 60*1000)
+	f("1H", 10, 1*60*60*1000)
+	f("1D", 10, 1*24*60*60*1000)
+	f("1Y", 10, 1*365*24*60*60*1000)
 }
 
 func TestDurationError(t *testing.T) {
diff --git a/parser.go b/parser.go
index 9b92dba..fcdf8ff 100644
--- a/parser.go
+++ b/parser.go
@@ -1341,6 +1341,7 @@ func (p *parser) parsePositiveDuration() (*DurationExpr, error) {
 			return nil, fmt.Errorf(`duration: parse error: %s`, err)
 		}
 	}
+
 	de := &DurationExpr{
 		s: s,
 	}
diff --git a/parser_test.go b/parser_test.go
index 4245469..dd7d523 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -309,6 +309,15 @@ func TestParseSuccess(t *testing.T) {
 	// funcName with escape chars
 	same(`foo\(ba\-r()`)
 
+	// duration converts to lower case
+	another(`{}[5m:3S]`, `{}[5m:3S]`)
+	another(`{}[5H:]`, `{}[5H:]`)
+	another(`{}[5M:3s]`, `{}[5M:3s]`)
+	another(`m[10M]`, `m[10M]`)
+	another(`m[10MS]`, `m[10MS]`)
+	another(`foo offset 5M`, `foo offset 5M`)
+	another(`foo offset 5S`, `foo offset 5S`)
+
 	// aggrFuncExpr
 	same(`sum(http_server_request) by ()`)
 	same(`sum(http_server_request) by (job)`)