diff --git a/Sources/Prometheus/MetricTypes/PromMetric.swift b/Sources/Prometheus/MetricTypes/PromMetric.swift
index 602786b..690ce84 100644
--- a/Sources/Prometheus/MetricTypes/PromMetric.swift
+++ b/Sources/Prometheus/MetricTypes/PromMetric.swift
@@ -13,6 +13,9 @@ public enum PromMetricType: String {
 }
 
 public enum Prometheus {
+    /// Default capacity of Summaries
+    public static let defaultSummaryCapacity = 500
+
     /// Default quantiles used by Summaries
     public static let defaultQuantiles = [0.01, 0.05, 0.5, 0.9, 0.95, 0.99, 0.999]
 }
diff --git a/Sources/Prometheus/MetricTypes/Summary.swift b/Sources/Prometheus/MetricTypes/Summary.swift
index c9bc48b..498d07e 100644
--- a/Sources/Prometheus/MetricTypes/Summary.swift
+++ b/Sources/Prometheus/MetricTypes/Summary.swift
@@ -1,4 +1,5 @@
 import NIOConcurrencyHelpers
+import NIO
 import struct CoreMetrics.TimeUnit
 import Dispatch
 
@@ -43,8 +44,11 @@ public class PromSummary<NumType: DoubleRepresentable, Labels: SummaryLabels>: P
     private let count: PromCounter<NumType, EmptyLabels>
     
     /// Values in this Summary
-    private var values: [NumType] = []
-    
+    private var values: CircularBuffer<NumType>
+
+    /// Number of values to keep for calculating quantiles
+    internal let capacity: Int
+
     /// Quantiles used by this Summary
     internal let quantiles: [Double]
     
@@ -60,9 +64,10 @@ public class PromSummary<NumType: DoubleRepresentable, Labels: SummaryLabels>: P
     ///     - name: Name of the Summary
     ///     - help: Help text of the Summary
     ///     - labels: Labels for the Summary
+    ///     - capacity: Number of values to keep for calculating quantiles
     ///     - quantiles: Quantiles to use for the Summary
     ///     - p: Prometheus instance creating this Summary
-    internal init(_ name: String, _ help: String? = nil, _ labels: Labels = Labels(), _ quantiles: [Double] = Prometheus.defaultQuantiles, _ p: PrometheusClient) {
+    internal init(_ name: String, _ help: String? = nil, _ labels: Labels = Labels(), _ capacity: Int = Prometheus.defaultSummaryCapacity, _ quantiles: [Double] = Prometheus.defaultQuantiles, _ p: PrometheusClient) {
         self.name = name
         self.help = help
         
@@ -74,6 +79,10 @@ public class PromSummary<NumType: DoubleRepresentable, Labels: SummaryLabels>: P
         
         self.count = .init("\(self.name)_count", nil, 0, p)
         
+        self.values = CircularBuffer(initialCapacity: capacity)
+
+        self.capacity = capacity
+
         self.quantiles = quantiles
         
         self.labels = labels
@@ -159,6 +168,9 @@ public class PromSummary<NumType: DoubleRepresentable, Labels: SummaryLabels>: P
             }
             self.count.inc(1)
             self.sum.inc(value)
+            if self.values.count == self.capacity {
+                _ = self.values.popFirst()
+            }
             self.values.append(value)
         }
     }
@@ -190,7 +202,7 @@ extension PrometheusClient {
         if let summary = summaries.first {
             return summary
         } else {
-            let newSummary = PromSummary<T, U>(summary.name, summary.help, labels, summary.quantiles, self)
+            let newSummary = PromSummary<T, U>(summary.name, summary.help, labels, summary.capacity, summary.quantiles, self)
             summary.subSummaries.append(newSummary)
             return newSummary
         }
diff --git a/Sources/Prometheus/Prometheus.swift b/Sources/Prometheus/Prometheus.swift
index 076f379..cb5ceb3 100644
--- a/Sources/Prometheus/Prometheus.swift
+++ b/Sources/Prometheus/Prometheus.swift
@@ -240,6 +240,7 @@ public class PrometheusClient {
     ///     - type: The type the summary will observe
     ///     - name: Name of the summary
     ///     - helpText: Help text for the summary. Usually a short description
+    ///     - capacity: Number of observations to keep for calculating quantiles
     ///     - quantiles: Quantiles to calculate
     ///     - labels: Labels to give this summary. Can be left out to default to no labels
     ///
@@ -248,6 +249,7 @@ public class PrometheusClient {
         forType type: T.Type,
         named name: String,
         helpText: String? = nil,
+        capacity: Int = Prometheus.defaultSummaryCapacity,
         quantiles: [Double] = Prometheus.defaultQuantiles,
         labels: U.Type) -> PromSummary<T, U>
     {
@@ -255,8 +257,7 @@ public class PrometheusClient {
             if let cachedSummary: PromSummary<T, U> = self._getMetricInstance(with: name, andType: .summary) {
                 return cachedSummary
             }
-            
-            let summary = PromSummary<T, U>(name, helpText, U(), quantiles, self)
+            let summary = PromSummary<T, U>(name, helpText, U(), capacity, quantiles, self)
             let oldInstrument = self.metrics.updateValue(summary, forKey: name)
             precondition(oldInstrument == nil, "Label \(oldInstrument!.name) is already associated with a \(oldInstrument!._type).")
             return summary
diff --git a/Tests/SwiftPrometheusTests/SummaryTests.swift b/Tests/SwiftPrometheusTests/SummaryTests.swift
index d835791..d6a4935 100644
--- a/Tests/SwiftPrometheusTests/SummaryTests.swift
+++ b/Tests/SwiftPrometheusTests/SummaryTests.swift
@@ -144,4 +144,21 @@ final class SummaryTests: XCTestCase {
         my_summary_sum{myValue=\"labels\"} 123.0
         """)
     }
+
+    func testStandaloneSummaryWithCustomCapacity() {
+        let capacity = 10
+        let summary = prom.createSummary(forType: Double.self, named: "my_summary", helpText: "Summary for testing", capacity: capacity, quantiles: [0.5, 0.99], labels: BaseSummaryLabels.self)
+
+        for i in 0 ..< capacity { summary.observe(Double(i * 1_000)) }
+        for i in 0 ..< capacity { summary.observe(Double(i)) }
+
+        XCTAssertEqual(summary.collect(), """
+        # HELP my_summary Summary for testing
+        # TYPE my_summary summary
+        my_summary{quantile="0.5", myValue="*"} 4.5
+        my_summary{quantile="0.99", myValue="*"} 9.0
+        my_summary_count{myValue="*"} 20.0
+        my_summary_sum{myValue="*"} 45045.0
+        """)
+    }
 }