diff --git a/src/main/scala/com/fulcrumgenomics/umi/CallDuplexConsensusReads.scala b/src/main/scala/com/fulcrumgenomics/umi/CallDuplexConsensusReads.scala
index 01ee0c6a3..55ad31513 100644
--- a/src/main/scala/com/fulcrumgenomics/umi/CallDuplexConsensusReads.scala
+++ b/src/main/scala/com/fulcrumgenomics/umi/CallDuplexConsensusReads.scala
@@ -28,6 +28,7 @@ import com.fulcrumgenomics.FgBioDef._
 import com.fulcrumgenomics.bam.OverlappingBasesConsensusCaller
 import com.fulcrumgenomics.bam.api.{SamOrder, SamSource, SamWriter}
 import com.fulcrumgenomics.cmdline.{ClpGroups, FgBioTool}
+import com.fulcrumgenomics.commons.collection.ParIterator.DefaultChunkSize
 import com.fulcrumgenomics.sopt.clp
 import com.fulcrumgenomics.commons.io.Io
 import com.fulcrumgenomics.commons.util.LazyLogging
@@ -108,6 +109,10 @@ class CallDuplexConsensusReads
  val maxReadsPerStrand: Option[Int] = None,
  @arg(doc="The number of threads to use while consensus calling.") val threads: Int = 1,
  @arg(doc="Consensus call overlapping bases in mapped paired end reads") val consensusCallOverlappingBases: Boolean = true,
+ @arg(doc="""
+          |The number of consensus groups to include in the buffer when using multiple threads.
+          |For input with very large family sizes, a smaller chunk size, such as 2*<threads> may be suitable.
+          """) val chunkSize: Int = DefaultChunkSize
 ) extends FgBioTool with LazyLogging {
 
   Io.assertReadable(input)
@@ -142,7 +147,7 @@ class CallDuplexConsensusReads
       maxReadsPerStrand   = maxReadsPerStrand.getOrElse(VanillaUmiConsensusCallerOptions.DefaultMaxReads)
     )
     val progress = ProgressLogger(logger, unit=1000000)
-    val iterator = new ConsensusCallingIterator(inIter, caller, Some(progress), threads)
+    val iterator = new ConsensusCallingIterator(inIter, caller, Some(progress), threads, chunkSize)
     out ++= iterator
     progress.logLast()
 
diff --git a/src/main/scala/com/fulcrumgenomics/umi/ConsensusCallingIterator.scala b/src/main/scala/com/fulcrumgenomics/umi/ConsensusCallingIterator.scala
index 449133c01..9b752669b 100644
--- a/src/main/scala/com/fulcrumgenomics/umi/ConsensusCallingIterator.scala
+++ b/src/main/scala/com/fulcrumgenomics/umi/ConsensusCallingIterator.scala
@@ -40,7 +40,7 @@ import com.fulcrumgenomics.util.ProgressLogger
   * @param caller the consensus caller to use to call consensus reads
   * @param progress an optional progress logger to which to log progress in input reads
   * @param threads the number of threads to use.
-  * @param chunkSize parallel process in chunkSize units; will cause 8 * chunkSize records to be held in memory
+  * @param chunkSize parallel process in chunkSize units; will cause chunkSize consensus groups to be held in memory
   */
 class ConsensusCallingIterator[ConsensusRead <: SimpleRead](sourceIterator: Iterator[SamRecord],
                                                             caller: UmiConsensusCaller[ConsensusRead],
@@ -67,10 +67,10 @@ class ConsensusCallingIterator[ConsensusRead <: SimpleRead](sourceIterator: Iter
       groupingIterator.flatMap(caller.consensusReadsFromSamRecords)
     }
     else {
-      ParIterator(groupingIterator, threads=threads).flatMap { rs =>
+      ParIterator(groupingIterator, threads=threads, chunkSize=chunkSize).map { rs =>
         val caller = callers.get()
         caller.synchronized { caller.consensusReadsFromSamRecords(rs) }
-      }.toAsync(chunkSize * 8)
+      }.toAsync(chunkSize).flatten
     }
   }