diff --git a/core/src/main/scala/sttp/model/headers/Range.scala b/core/src/main/scala/sttp/model/headers/Range.scala
index 9bbaebf0..a31df891 100644
--- a/core/src/main/scala/sttp/model/headers/Range.scala
+++ b/core/src/main/scala/sttp/model/headers/Range.scala
@@ -13,11 +13,11 @@ case class Range(start: Option[Long], end: Option[Long], unit: String) {
   def toContentRange(fileSize: Long, unit: String = ContentRangeUnits.Bytes): ContentRange =
     ContentRange(unit, start.zip(end).headOption, Some(fileSize))
 
-  val contentLength: Long = end.zip(start).map(r => r._1 - r._2).headOption.getOrElse(0)
+  val contentLength: Long = end.zip(start).map(r => r._1 - r._2 + 1).headOption.getOrElse(0)
 
   def isValid(contentSize: Long): Boolean =
     (start, end) match {
-      case (Some(_start), Some(_end)) => _start < _end && _end < contentSize
+      case (Some(_start), Some(_end)) => _start <= _end && _end < contentSize
       case (Some(_start), None)       => _start < contentSize
       case (None, Some(_end))         => _end < contentSize
       case _                          => false
@@ -56,7 +56,7 @@ object Range {
 
   private def validateRange(range: Range): Boolean =
     (range.start, range.end) match {
-      case (Some(start), Some(end)) => start < end
+      case (Some(start), Some(end)) => start <= end
       case (Some(_), None)          => true
       case (None, Some(_))          => true
       case _                        => false
diff --git a/core/src/test/scala/sttp/model/headers/RangeTest.scala b/core/src/test/scala/sttp/model/headers/RangeTest.scala
index e304aca0..b2a8053c 100644
--- a/core/src/test/scala/sttp/model/headers/RangeTest.scala
+++ b/core/src/test/scala/sttp/model/headers/RangeTest.scala
@@ -6,6 +6,11 @@ import sttp.model.ContentRangeUnits
 
 class RangeTest extends AnyFlatSpec with Matchers {
 
+  it should "properly parse the Range header of length one" in {
+    val actual = Range.parse("bytes=10-10")
+    actual shouldBe Right(List(Range(Some(10), Some(10), ContentRangeUnits.Bytes)))
+  }
+
   it should "properly parse simplest Range header" in {
     val actual = Range.parse("bytes=200-1000")
     actual shouldBe Right(List(Range(Some(200), Some(1000), ContentRangeUnits.Bytes)))
@@ -109,7 +114,7 @@ class RangeTest extends AnyFlatSpec with Matchers {
   }
 
   it should "calculate content length" in {
-    Range(Some(100), Some(200), ContentRangeUnits.Bytes).contentLength shouldBe 100
+    Range(Some(100), Some(200), ContentRangeUnits.Bytes).contentLength shouldBe 101
   }
 
   it should "map RangeValue to content type" in {