diff --git a/spec/std/fiber/execution_context/parallel_spec.cr b/spec/std/fiber/execution_context/parallel_spec.cr
index cb358577f453..f049073b88a7 100644
--- a/spec/std/fiber/execution_context/parallel_spec.cr
+++ b/spec/std/fiber/execution_context/parallel_spec.cr
@@ -7,14 +7,16 @@ describe Fiber::ExecutionContext::Parallel do
     mt.size.should eq(0)
     mt.capacity.should eq(2)
 
-    expect_raises(ArgumentError, "needs at least one thread") do
+    expect_raises(ArgumentError, "Parallelism can't be less than one") do
       Fiber::ExecutionContext::Parallel.new("test", maximum: -1)
     end
 
-    expect_raises(ArgumentError, "needs at least one thread") do
+    expect_raises(ArgumentError, "Parallelism can't be less than one") do
       Fiber::ExecutionContext::Parallel.new("test", maximum: 0)
     end
 
+    # the following are deprecated constructors
+
     mt = Fiber::ExecutionContext::Parallel.new("test", size: 0..2)
     mt.size.should eq(0)
     mt.capacity.should eq(2)
@@ -24,14 +26,14 @@ describe Fiber::ExecutionContext::Parallel do
     mt.capacity.should eq(4)
 
     mt = Fiber::ExecutionContext::Parallel.new("test", size: 1..5)
-    mt.size.should eq(1)
+    mt.size.should eq(0)
     mt.capacity.should eq(5)
 
     mt = Fiber::ExecutionContext::Parallel.new("test", size: 1...5)
-    mt.size.should eq(1)
+    mt.size.should eq(0)
     mt.capacity.should eq(4)
 
-    expect_raises(ArgumentError, "needs at least one thread") do
+    expect_raises(ArgumentError, "Parallelism can't be less than one.") do
       Fiber::ExecutionContext::Parallel.new("test", size: 0...1)
     end
 
diff --git a/src/fiber/execution_context/parallel.cr b/src/fiber/execution_context/parallel.cr
index 2e041003936f..2052ff6588ed 100644
--- a/src/fiber/execution_context/parallel.cr
+++ b/src/fiber/execution_context/parallel.cr
@@ -69,56 +69,46 @@ module Fiber::ExecutionContext
 
     @parked = Atomic(Int32).new(0)
     @spinning = Atomic(Int32).new(0)
-    @size : Range(Int32, Int32)
+    @capacity : Int32
 
     # :nodoc:
     protected def self.default(maximum : Int32) : self
-      new("DEFAULT", 1..maximum, hijack: true)
+      new("DEFAULT", maximum, hijack: true)
     end
 
-    # Starts a context with a *maximum* number of threads. Threads aren't started
-    # right away but will be started as needed to increase parallelism up to the
-    # configured maximum.
+    # Starts a context with a *maximum* parallelism. The context starts with an
+    # initial parallelism of zero. It will grow to one when a fiber is spawned,
+    # then the actual parallelism will keep increasing and decreasing as needed,
+    # but will never go past the configured *maximum*.
     def self.new(name : String, maximum : Int32) : self
-      new(name, 0..maximum)
+      new(name, maximum, hijack: false)
     end
 
-    # Starts a context with a *maximum* number of threads. Threads aren't started
-    # right away but will be started as needed to increase parallelism up to the
-    # configured maximum.
+    @[Deprecated("Use Fiber::ExecutionContext::Parallel.new(String, Int32) instead.")]
     def self.new(name : String, size : Range(Nil, Int32)) : self
-      new(name, Range.new(0, size.end, size.exclusive?))
+      new(name, size.exclusive? ? size.end - 1 : size.end, hijack: false)
     end
 
-    # Starts a context with a minimum and maximum number of threads. Only the
-    # minimum number of threads will be started right away. The minimum can be 0
-    # (or nil) in which case no threads will be started. More threads will be
-    # started as needed to increase parallelism up to the configured maximum.
+    @[Deprecated("Use Fiber::ExecutionContext::Parallel.new(String, Int32) instead.")]
     def self.new(name : String, size : Range(Int32, Int32)) : self
-      new(name, size, hijack: false)
+      raise ArgumentError.new("invalid range") if size.begin > size.end
+      new(name, size.exclusive? ? size.end - 1 : size.end, hijack: false)
     end
 
-    protected def initialize(@name : String, size : Range(Int32, Int32), hijack : Bool)
-      @size =
-        if size.exclusive?
-          (size.begin)..(size.end - 1)
-        else
-          size
-        end
-      raise ArgumentError.new("#{self.class.name} needs at least one thread") if capacity < 1
-      raise ArgumentError.new("#{self.class.name} invalid range") if @size.begin > @size.end
+    protected def initialize(@name : String, @capacity : Int32, hijack : Bool)
+      raise ArgumentError.new("Parallelism can't be less than one.") if @capacity < 1
 
       @mutex = Thread::Mutex.new
       @condition = Thread::ConditionVariable.new
 
       @global_queue = GlobalQueue.new(@mutex)
-      @schedulers = Array(Scheduler).new(capacity)
-      @threads = Array(Thread).new(capacity)
+      @schedulers = Array(Scheduler).new(@capacity)
+      @threads = Array(Thread).new(@capacity)
 
       @rng = Random::PCG32.new
 
       start_schedulers
-      start_initial_threads(hijack)
+      @threads << hijack_current_thread(@schedulers.first) if hijack
 
       ExecutionContext.execution_contexts.push(self)
     end
@@ -130,7 +120,7 @@ module Fiber::ExecutionContext
 
     # The maximum number of threads that can be started.
     def capacity : Int32
-      @size.end
+      @capacity
     end
 
     # :nodoc:
@@ -156,19 +146,6 @@ module Fiber::ExecutionContext
       end
     end
 
-    private def start_initial_threads(hijack)
-      offset = 0
-
-      if hijack
-        @threads << hijack_current_thread(@schedulers[0])
-        offset += 1
-      end
-
-      offset.upto(@size.begin - 1) do |index|
-        @threads << start_thread(@schedulers[index])
-      end
-    end
-
     # Attaches *scheduler* to the current `Thread`, usually the process' main
     # thread. Starts a `Fiber` to run the scheduler loop.
     private def hijack_current_thread(scheduler) : Thread
@@ -221,7 +198,7 @@ module Fiber::ExecutionContext
     # Picks a scheduler at random then iterates all schedulers to try to steal
     # fibers from.
     protected def steal(& : Scheduler ->) : Nil
-      return if size == 1
+      return if capacity == 1
 
       i = @rng.next_int
       n = @schedulers.size